Ranking System of the Galactic Empire

Using "A New Hope", "Empire Strikes Back" and wookiepedia as
sources gives the following titled [and assumed] ranks:
1. Grand Admiral
2. Grand Moff
3. Moff
4. Lord
5. Admiral
[6. Vice Admiral]
[7. Rear Admiral]
10. Captain
12. Ensign;
bearing in mind that in the Clone Wars and in the Rebel Alliance,
the rank of 'Captain' is below both 'Commander' and 'General':
'Commander' being apparently equivalent to anything between a
Colonel and a Major-General in Terran armies.

Ensign is included in this list as, apparently,
'Lieutenant' is the lowest commissioned Imperial rank.
An explanation follows in that it is possible that the
three 'Warrant Officer' grades are both promotions from
lower ranks and starting grades for officers graduating
below the 'Lieutenant' requirement. An 'Ensign' may be a
Warrant Officer in command of a section, particular shift
or a platoon. It is suggested that the rank of 'Captain'
does exist, but has two grades.

With other actual quoted ranks being (also
using "A New Hope" and wookiepedia as sources):

5. Marshall
6. General
7. Colonel
8. Major
9. Commander
11. Lieutenant
12. Senior-
13. Master-
14. Warrant Officer
15. Sergeant
16. Staff Corporal
17. Corporal
18. Trooper

As is apparent there is a large discrepancy between
the positions of certain ranks in relation to one
another, especially as the insignia used in the
Imperial Navy and Captain Piett's promotion
to Admiral would suggest otherwise.

It could obviously be suggested that there are in fact
two chains of command in existence, but both Imperial
Army and Naval divisions in each Sector

(of which there are between 20 and 1024; the priority regions
set up by Palpatine towards the end of the Clone Wars
or the sector system used by the Old Republic)

are all under the command of the particular system's
governor, or 'Moff', so in terms of each sector
they are under the same chain-of-command. This also
allows for "official" ranks to reflect the vast
size of the Imperial military.

The simplest answer is that the Imperial Army ranks
reflect the number of troops (and Stormtroopers
in particular are seen being led by Naval officers)
at an officer's disposal while, and this is basically
indicated by the seeming lack of consistency in the Naval
insignia used by officers, the flag ranks used in the
Navy likely represent a position in the sector
fleet but has little relation to one's overall rank
in the Imperial Military.

The types of insignia (below) used probably represent
the actual order of promotion as opposed to
the actual rank:

Six double squares (vertical) = Moffs

Three double squares = Flag Officers

Two double squares = Command Officers

Four horizontal squares = Junior Officers

One double square = Warrant Officers

No insignia = Ensign

The actual uniforms used probably make more sense
in making sense of the ranks used and clear up
much of the confusion quoted above, it should
be remembered that each sector could cover
anywhere from a million to maybe over a trillion
troops and personnel. Duties are given for each
uniform along with typical ranks:

White = Grand Admiral, Sector over-commander

Grey = Moff, Sector governor

Olive = General, General staff

Black = Lieutenant to Commander, Navy line officer

Pale/Creamy Grey = Staff Corporal to Senior Warrant
Officer, Technical staff

Armour (black) = Corporal and above, Specialists

Armour (white) = Enlisted, Infantry