X-kill-
You're known as a TPU, or Troop Program Unit, member. There's an actual slot, unit, command, etc that "owns" you. When you're away at school, or whatever, you still belong to them. If they are activated, you hookup with them when you can.
IMA's, or Idividual Mobiliization Augmentees, are Reserve members who are assigned to an actual Active Duty unit to reinforce them during mobilization. If the 101st goes to war, then those IMAs assigned to it get called up and fill slots that normally are unfilled because the Army is generally only 75-80% strength in units.
There are also "floaters". I forgot the actual designation, but they go from place to place and unit to unit just doing whatever it is. Things like being a OC for a couple of weeks in Italy, then next year going to Team Spirit in Korea. During wartime they usually get called-up to fill shortages in their specialties, but aren't assigned to a specific place or unit ahead of time. They're basically the flex team.
Nearly all positions in the Guard are TPU because of the nature of the Guard. Think of it the same as active duty units. If you're in the 101st, and the 10th Mountain gets deployed, you don't go because your unit isn't going. If your unit goes, you go with it. Simple as that. Now the Regular Army has more leway in stealing folks from one place to fill shortages in another that's deploying, but for the most part you'll do whatever your unit's doing.
Ross