There’s also administrative separation, which is essentially a non-punitive discharge. For example, in the Army, if certain officers (CW3-CW5, or a Captain, Major, or Lieutenant Colonel) are not promoted after twice being considered by a promotion board, they will be separated unless they are within 2 years of being able to retire or their senior (superior officer) rater gets special approval to retain them. Generally, though, if the officer being considered for promotion has either vital skills or vital usefulness that warrants going through the ordeal of getting special approval to retain them, they probably would have been promoted in the first place.
The monthly income tax exclusion for officers in a combat zone is limited to the highest rate of enlisted pay (not counting any imminent danger or hostile fire pay recieved).