Bottom line
If you are an enlisted Reserve Marine, this section covers three things: the career help available to you (counseling, record review, FitRep evaluation, training and education options), how Non-Prior Service (NPS) tours work, and how SNCO tours and billet retention work by grade. The headline numbers: the normal SMCR tour for a sergeant major or first sergeant in an 8999 billet is three years, extendable one year at a time by DC M&RA (RA). There are no tour-length limits for gunnery sergeants and below.
What it means for you
Most of this is opportunity and machinery, not a deadline. You have a career counselor you can actually call or email (numbers below). If you came in NPS, you were recruited to a specific unit and PMOS, you owe a mandatory drilling period on your contract, and after you finish it you can move to the IRR for whatever time is left.
The tour rules matter most if you are a senior SNCO. Sergeant major and first sergeant 8999 billets run three years with one-year extensions. Once you are senior to your billet's grade, you can be kept in place until a qualified replacement shows up, a more junior Marine in the unit is promoted into the grade and MOS, or you are promoted up. As a general rule, an enlisted billet can be filled by a Marine one grade above or one grade below the billet grade.