Scale of Imperial Military

Due to the scale of the Imperial Military, having to span the galaxy of thousands of worlds, ranks and administration positions higher than four-star ranks are needed.

Moff

As Army and Naval organisation becomes larger than any of Earths militaries, another level of organisation is required. Moffs held authority over the immense military resources of a Sector Fleet or Sector Group within the Imperial Navy, as well as a Sector army. Moffs are more a political position risen from military or plantary positions.

• Grand Moff:
  Are governors of Oversectors, which were grouped Star Systems and Sectors that demonstrated signs of unrest.
• Moffs:
  Controlled a sector of 50 inhabited Star Systems, had ultimate authority over the deployment of all military forces within their sector. Formal superiors of all Planetary Governors, paramilitary and Imperial Intelligence, were both supposed to be channeled through the Moff's office at Sector level.
• Governor:
  Multiple levels of positions with authority over a Planet, Moon, Colony, Station, Province, or other sub-national body.
• Governor:
  Multiple levels of positions with authority over a Star System. They report to the Sector Moff or Sector Adjutant, the Governor would be given control of some military within their System.
• Legate:
  Are in charge of an individual Planet within a Star System. They report to the System Governor, and sometimes work with the local government.
• Prefect:
  Are in charge of a District on a Planet, Moon, Colony, or Station.
• Instance of High Moff after the fall of the Empire, alternatively, a Supreme Moff, incharge of a quadrant or large section of the the galaxy, or possibly in charge of a section of government.

OF-11 or 6-star ranks

• High General: Instead of Surface Marshal title
  Commander of Imperial Army forces within a Sector army. Along with the Navy title of High Admiral, this title was typically borne by the Moff or Grand Moff, though in some Sectors, it was delegated to a senior subordinate.
• High Admiral:
  Commander of Navy forces within an Imperial Sector Group. Along with the Army title of Surface Marshal, this title was typically borne by the Moff or Grand Moff, though in some Sectors it was delegated to a senior subordinate.

OF-10 or 5-star ranks

• Surface General: Instead of High General title
  Normally controlled a Systems Army, a largely bureaucratic level of command immediately subordinate to the Surface Marshal in charge of a Sector Army.
• Fleet Admiral:
  Fleet Admiral may not be a formal line rank but a position that could be held by Admirals or Vice Admirals. The title of Fleet Admiral was accorded to the commanding officer of a major fleet within a Sector Group. A Fleet Admiral answered directly to the High Admiral in charge of Navy operations within their Sector

OF-5a/6b or Line ranks

Line ranks are a senior OF-5 rank to distinguish between the commander of a line and a componet of a line. Or a title to distinguish a divisional command level officer without giving them flag rank. Or an extra rank for extra units: Legion, Battlegroup, Line of Battle, or Star Wing.

• High Colonel:
  Title given in the Imperial Army to the officer commanding a Battlegroup, as well as the title given in the Stormtrooper Corps to the officer commanding a Legion. An additional rank grade inserted on a level between Colonel and Brigadier.
• Line Captain:
  A Imperial-class Star destroyer by itself, is considered a line of battle. The ships captain is also in overall command of Stormtrooper, Flight, and other divisions that are by themselves commanded by an OF-5 officer.
• Star Commodore:
  Senior command officer on ships or commands larger than the Imperial-class or Line of battle.

Warrant Officer

A military officer, usually a skilled technician or pilot, intermediate in rank between a noncommissioned officer and a commissioned officer, having authority by virtue of a warrant.

• Sergeant major:
  Is the origin of Major and Major General, ranking below lieutenant and droping the Sergeant.

Armies

• High General:
  A ranks explicitly introduced in The Imperial Sourcebook, like "High Admiral" The ranks may really be equal in seniority, but the sourcebook does not say so. - SWTC
• Surface Marshal:
  an adaptation of "Field Marshal" on Earth; this term was introduced in The Imperial Sourcebook. - SWTC
• High Colonel:
  an unusual army officer rank inserted above Colonel by The Imperial Sourcebook; this level is either synonymous with or junior to a Brigadier. - SWTC
• Staff Corporal:
  not in widespread use; non-standard. - SWTC
• Staff Sergeant / Sergeant Major:
  A "staff sergeant" may sometimes be called a "sergeant major" (an American variation of common English terminology). The latter term is used in The Imperial Sourcebook for the subofficer who is second-in-command of a platoon. - SWTC
• [Lance] Corporal:
  In artillery units a Corporal is known as a Bombardier, and a Lance Corporal as a Lance Bombardier. - SWTC
• Private:
  Some specialist army units use alternative ranks equivalent to Private: Trooper in armoured units; Gunner in artillery; Signalman in signals; Musician in bands; etc. - SWTC

Navies

• High Admiral:
  A rank explicitly introduced in The Imperial Sourcebook; they are inserted into the general convention because STAR WARS armed forces are larger in scope than Earthly equivalents, thus requiring more levels of higher officers. Ideally the ranks of High Admiral and High General ought to be equally senior, but The Imperial Sourcebook makes a swap with Surface Marshal and Fleet Admiral. This may be a mistake. - SWTC
• Fleet Admiral:
  identical to "Admiral of the Fleet" on Earth. The form used here eliminates superfluous words. "Fleet Admiral" is also used throughout roleplaying game and other spin-off references from The Imperial Sourcebook onwards. - SWTC
• Line Captain:
  unusual navy officer rank inserted above [naval] Captain by The Imperial Sourcebook; they are probably at the same level as "High Colonel" in the same book. A line captain must be either synonymous with or junior to a Commodore. - SWTC
• Midshipman / Ensign:
  These two terms refer to the most junior commissioned naval officers, who have finished their academy training and are earning experience aboard an active warship. A "midshipman" may also refer to a naval officer cadet who has no space experience and is still studying at his academy; however the term "ensign" is never applied to such a trainee. [This table is a consolidation of American and non-American terminology, both of which are used in the STAR WARS literature.] - SWTC

Fighter Forces

• American Air Force Ranks and Org:
  USAF follows Army titles for ranks. A USAF Wing contains 2 or more Groups.
• British/ Commonwealth Air Force Ranks and Org:
  Air Force follows Naval titles for ranks. An Air Force Group contains 2 or more Wings. Since the Starfighter Core is part of the Imperial Navy, pilot ranks should follow Naval rank structure.
• Starfighter titles:
  Most of the military titles for fighter forces are heavily geocentric, involving references to "air" and other phenomena which are specific to pre-space flight. For adaptation to STAR WARS I replace or strip off these parts of the terms. "Spaceman" could be used as a concise alternative to "spacecraftsman." However a character called Air Marshal Von Asch in SW Tales #5 has a fully terrestrial title. - SWTC
• Pilots:
  Only commissioned officers in the starfighter force actually fly fighters. Non-commissioned personnel are engaged in starfighter support activity. The highest officers are not often involved in combat; they will be involved in overall strategy and command. - SWTC
• High Marshal:
  conjectural higher starfighter force rank, based on extrapolation from naval practice; equivalent to naval High Admiral. - SWTC
• Starfighters & Army:
  When fighter operations are temporarily dependent on the Army's surface bases, pilots sometimes use equivalent army ranks in verbal forms of address. The army Captain title applied to a Flight Lieutenant should never be confused with the superior starfighter rank of Group Captain, which equates to a naval Captain or an army Colonel. This seems to be the only charitable explanation for the occasional use of "captain" for a pilot officer inferior to a Commander in certain comics, novels and computer games. Ditto for any application of "major", "colonel" or "general" to pilots. - SWTC
• Higher Command:
  Divisions above a numbered division will not just include star fighters, but also include formations for logistics, bombers, shuttles etc. - SWTC

Stormtrooper

Stormtroopers seem to have lesser number of ranks, similar to British and Commonwealth, there are no intermediate ranks between Private, Assistant Squad Leader, Squad Leader and Platoon Sergeant.
I summise, Black as Trooper, White as Squad Leader/ Sergeant, and Orange as a Senior Non-Commissioned Officer or Warrant Officer. Blue, Red and Yellow would signifiy Commissioned Officers.

• No Pauldron:
  Trooper.
• Black Pauldron color:
  Trooper, Squad Leader.
  If the above [White color] reasoning is correct then those with black pauldrons must be the actual troops. It is significant that troops with black pauldrons are the most numerous [in ANH:SE]. This abundance of black supports the notion that these are the basic stormtroopers. - SWTC
• White Pauldron color:
  Squad Leader, Sergeant.
  Given that the white pauldron is a patch on a black base (whereas the black pauldron seems to be merely the absence of a patch altogether) I infer that white is for subofficers. STAR WARS Visual Dictionary specifically says that they're sergeants. - SWTC
• Orange Pauldron color:
  Squad Leader, Sergeant, Senior Sergeant, Senior Non-Commissioned Officer, Warrant Officer, Commissioned Officer.
• Red/orange Pauldron color:
  This seems to be the most senior colour known. It may indicate a low or mid-ranking officer; or it might be a high NCO. All the stormtroopers so far seen on Tatooine took orders from those who wore this colour. Their rarity relative to lower ranks suggests that they may be equivalent to Captain, Lieutenant or Second Lieutenant. - SWTC
• Sandtroopers:
  Sandtroopers wear colored pauldrons as a designation of rank. A stormtrooper sergeant's pauldron is white while an enlisted stormtrooper's pauldron is all black. Squad leaders, who lead units of seven troopers, wear orange shoulder pauldrons.
• Clone Troopers:
  White: Trooper
  Green: Sergeant - Squad leader
  Blue: Lieutenant - Platoon commander
  Red: Captain - Company commander
  Yellow: Commander - Regiment commander
  
• High Colonel:
  Title given in the Stormtrooper Corps to the officer commanding a Legion. It is an unusual army officer rank inserted above Colonel, a rank equivalent or junior to a Brigadier.
  A title to distinguish a divisional command level officer without giving them flag rank. In the Stormtrooper Corps this would ensure that a Legion level officer in command of an Imperial Star Destroyer's Stormtrooper contingent would not outrank the ship's captain.

ISB

• Led by a Bureau Chief under which served Deputy Director Harus Ison and Colonel Yularen.
• Deputy Director Harus Ison, Colonel Wullf Yularen, Major Oniye Namada, Captain Jaller Obrim, Lieutenant Adion Lang, Lieutenant Jeffren Brek, Corporal Vandolay, Agent Tian Chyler, Agent Elena Shelvay

COMPNOR

• Led by a director (commission General) and a deputy director (select commitee). Each arm of COMPNOR had its own director. Each arm is then divided by sector then into COMPNOR sub division. COMPNOR director reports to Emporer or Imperial Advisors

Sources

• SWTC
  Original table and comments copied from Star Wars Technical Commentaries and can be found here.