Who is asking and in what context?
While he can identify himself down to the platoon (or similar unit in another service) in many circumstances there would be no reason to do so.
If a civilian asks, as a general interest question, he could ID himself with the type of unit he was in (Space Navy, Army, Colonial Marines, whatever) or could id himself with the overall unit they might have heard of. "I was in the Fifth Spaceborne Infantry" (or whatever).
Unless they understand the military there's no real reason to ID himself in any more detail.
Now, talking to another military person he might get more detailed, especially if they are comparing notes as to where they served, who they served with, etc. Then you'd get into: Overall command, subunit, subunit, down to whatever level is needed. But, as a general rule, he wouldn't get into that much detail unless there is a reason. (Like, which Platoon or Company he was in, unless they want to know if he knows someone in that Company)