Bottom line
This section sets the ground rules for SELRES duty: how many paid IDT periods you can do in a year, how far you can be made to commute, the appearance and weight standards you have to keep, and what happens to your training and your seat if your unit is deactivated, redesignated, or relocated. The hard caps to remember: 48 paid IDT periods per fiscal year, and a 150-mile reasonable-commuting limit a mandatory participant can be required to travel to a unit.
What it means for you
If you are in an SMCR unit or you are an IMA, this is your baseline. You cannot be ordered past 48 paid IDT periods in a fiscal year, and certain extra training periods (ATP, AFTP, RMP) do not count against that 48. You cannot be forced to commute past the published limits. You have to meet height, weight, and appearance standards to be ordered to active duty. And if your unit goes away, you have defined options (retrain, transfer to another unit, move to the IRR, inter-service transfer, or discharge), each with consequences for your incentives and your Reserve Montgomery GI Bill (MGIB-SR). IRR and Standby Reserve Marines are covered in their own chapters.