Not sure if this applies to the guard or reserves but this is what I got:
AFI36-3212
1.10. Medical Hold. The Medical Standards Branch (HQ AFPC/DPAMM) may place a member on
medical hold when he or she is within 60 days of the scheduled non-disability separation or retirement
date and undergoing disability processing. HQ AFPC Service Retirements, AF/DPOB (for Colonels and
Colonel-selects), or the MPF must revoke the non-disability retirement or discharge order before its effec-
tive date.
hope this helps, what it basically means is they will extend you as far as they need to complete your MEB process.