Bottom line
This section is the rulebook for moving between Reserve categories: SMCR to IRR, IRR to SMCR, into the AR program, to another military service, or back to active duty. The rules split by whether you are enlisted or an officer, and they get stricter if you are a mandatory participant (you still owe drilling time on your contract). If you are a mandatory participant, you cannot just leave the drilling pool. Most moves need a written request and a specific approval level, often COMMARFORRES (G-1). There is also a separate path for a religious missionary obligation that conflicts with training.
What it means for you
If you want to change units or change status, your starting question is: am I still on the hook for a drilling obligation? If yes, you are a mandatory participant, and most exits run through a written request, an approval at COMMARFORRES level, and a check that your contract has enough time left to cover what you still owe. If your obligation is done, transfers are easier but still need your gaining unit's commander to want you and the right approval to sign off.
Watch the approval levels. They are not interchangeable. A request that has to be signed by COMMARFORRES (G-1) cannot be signed by anyone below that. For inter-service transfers, the DD 368 cannot be signed below the level the rule names, and there is a 30-day clock on the answer. Move physically (change your permanent address out of commuting range) and you do not just drop off the rolls. You get transferred to a unit near your new home, or to the IRR if no unit is in range.