Bottom line
The Marine Corps keeps the Ready Reserve screened so it knows who is ready to mobilize before a war or national emergency. You will be screened at least once every three years, and your unit also screens you before any deployment. IRR Marines have to confirm their status by performing Muster Duty (MD) or by responding by mail or electronically. If you are ever identified for mobilization, there are specific paths to request a deferment, delay, or exemption (DD&E) for things like hardship, conscientious objection, or certain civilian roles. When you demobilize, you go through the Transition Readiness Seminar.
What it means for you
Screening is how the Marine Corps confirms you are findable, fit, and qualified to be called up. The big practical asks for you: keep your contact info, dependency status, and qualifications current, and respond when you are screened. IRR Marines carry the most direct responsibility here, since you confirm your status through a muster or a mail/electronic screening rather than at drill. If a recall would genuinely break something in your life or work, this section is the menu of exceptions and who decides them. None of these are automatic. You have to request them, and someone above you has to approve them.