What it means for the recently separated officers who did not join the SELRES/PIRR: You are most likely* in the IRR if you:
1. Volunteered for it (ie obtained a reserve commission--requires positive action to be scrolled)
2. Are required to be in the IRR by reason of unfulfilled Military Service Obligation (< 8 years total service) or because you received separation pay (which requires at least 3 yrs IRR service).
3. Otherwise have a reserve commission in the ready reserve (i.e. you have not resigned your reserve commission, nor was your commission revoked (eg by twice failure of selection for promotion), nor did you transfer to the retired reserve).
Retirees are not in the ready reserve, so this order does not affect them.
* I say most likely because that is what is supposed to happen. But I have witnessed on at least 2 occasions where AFPC failed to place separating people in the IRR, even though they were required to do so by law.
BLAB: If you separated at the end of your UPT commitment, you are most likely not in the IRR. If you want to be sure you can check Milconnect. You'll need a DS logon, though.