The Imperial military is split into three major combat arms. They supplement and support each other as much as possible, but they are as different as army, navy, and air force. While they have the same rank structure, their jobs are completely different.
Planetary Surface Forces are the ground forces. They are the ones who go down onto a planetary surface – or under it – and root out hostile forces. Pretty much all military personnel start in Planetary Surface Forces – the others recruit by transfer, which generally involves starting anew.
Strategic Space Forces are the people that crew the largest ship classes and their auxiliaries. Post and Capital ships are crewed by Strategic Space Forces.
Tactical Space Forces are the people that crew the smaller ship classes, from Convoy hulls on down through Large and Small Cruisers, Destroyers, Cutters, and Fighters. They probably have the highest level of prestige, and definitely have the highest casualty levels.
Also part of the Imperial military are the Shield units. They’re not a major branch, although if you think in single planet terms their numbers are vast. Their reason for existence is the power and coherence of Imperial energy weapons. A shot that can damage a capital ship will obliterate an unshielded planet or a small civilian ship without noticing, and they are capable of doing it at ranges exceeding the size of most galaxies. Their job is after any battle involving military units to make certain there are as few shots as possible remaining from the battle. Their ships are essentially massive metallic rocks with hull charge, protected by capitol ship quality shields, and they intentionally move into the path of shots that have missed and absorb them. It’s not a particularly dangerous job, but has high prestige in certain parts of Imperial society.
Two other much smaller units are the Merlon’s Eyes and the Merlon’s Fingers. The Eyes are intelligence gatherers, the Fingers are assassination and sabotage operatives. They are each composed exclusively of operant Guardians, and neither is a military unit in any kind of traditional sense because each is composed of individuals with other military assignments who also do Eye or Finger work (Their civilian equivalents, the Guardian’s Eyes and the Guardian’s Fingers, are full time operatives but much fewer in number). Both groups are aimed at external enemies, and strive to have their targets remain unaware that they have done their work, completely unaware that their intelligence is compromised or sincerely believing that the unfortunate event that hamstrung them was the result of some other cause.
There are also a plethora of military forces in the employ of major families or powerful individuals. They tend to be elites (“Imperial blue and gold” is a phrase of some scorn among them). Individually, none of them is large enough to fight the Empire, but if they ever banded together they might be able to defeat Imperial forces. The rank structure for most private forces follows the Imperial structure closely.