Bottom line
This section covers the basics of how you come into the Reserve and what you owe in return. If you came in on or after 1 June 1984, you carry an eight-year Military Service Obligation (MSO). You also have a contractual drilling obligation with a date attached to it, the Mandatory Drill Participation Stop Date (MDPSD). And if you want to lateral move into a ground combat arms MOS, you have to meet the MOS classification standard first.
What it means for you
Two clocks run from the day you come in. The first is your MSO: eight total years of obligated service, and any part you do not serve on active duty or ADT gets served in the Ready Reserve. The second is your MDPSD: the contractually binding date by which you have met your initial drilling obligation with an SMCR unit. Once you hit your MDPSD, you can serve the rest of your MSO in the IRR or keep drilling in the SELRES.
One thing to watch: if your unit gets an order to mobilize before you reach your MDPSD, you mobilize and deploy with the unit. Wanting to roll into the IRR does not get you out of that.