Bottom line
This is the map of the whole Reserve: how it is structured and where you fit. The Marine Corps Reserve breaks into three big buckets: the Ready Reserve, the Standby Reserve, and the Retired Reserve. The Ready Reserve is where most active reservists live, and it splits into the Selected Reserve (SelRes) and the Individual Ready Reserve (IRR). Nothing here is an action item. It is the orientation chapter the rest of the Order builds on.
What it means for you
Your category decides almost everything else: whether you train, whether you get paid, whether you earn retirement points, and whether you can be promoted. If you drill with a unit, sit in an IMA billet, or serve on the AR program, you are in the SelRes. If you are in the manpower pool without a drilling billet, you are in the IRR. Both of those sit under the Ready Reserve, which is the part of the force subject to mobilization in war or national emergency. The Standby Reserve is a lighter-touch status (mostly no training, limited or no points), and the Retired Reserve is where you land after a qualifying career. Find your bucket below, and the rest of the manual will make more sense.