Bottom line
This section covers how your MOS drives your billet, how an MOS gets assigned, changed, or voided, and what happens if you are not MOS qualified for the billet you fill. The rule that bites: if you are a prior service (PS) Marine in the SMCR and you are not MOS qualified for your billet, you have 12 months from the date retraining is authorized to get qualified. Miss it and you can be involuntarily moved to the IRR. It also states the policy that female Marines are recruited and assigned the same as everyone else under current DoD and Marine Corps policy.
What it means for you
Your billet should match your primary MOS (PMOS) and pay grade. In most cases you can be assigned a billet one pay grade above or below your current grade. If you are sitting in a billet your PMOS does not match, you are expected to fix it: do an Inter-Unit Transfer (IUT), reclassify, or accept a reassignment. Ignore it and you can be pushed to the IRR.
If you are PS and not MOS qualified for your billet, the clock starts the day retraining is authorized. You have 12 months to begin a formal school or on-the-job training program. The unit, not you, is responsible for registering you for formal school. You should be counseled 6 months before that 12-month mark and warned that transfer to the IRR is possible.