Bottom line
TRS is a premium-based health plan that lets qualified SELRES Marines and their families buy TRICARE Standard and Extra coverage at a reduced rate. Only SELRES Marines joined to an SMCR unit or an IMA billet can enroll. The same deductibles and cost shares that apply to active duty Marines apply here, which makes it a competitive option. If you transfer to the IRR, your TRS eligibility ends automatically.
What it means for you
If you drill in the SELRES under an SMCR unit or IMA billet, you can pay a premium and put yourself and your family on TRICARE coverage that works much like an active duty Marine's. You can start coverage on the first day of any month. There is one premium for member-only coverage and one for member-and-family coverage. The moment you move to the IRR, the benefit stops. A few other categories are locked out entirely (see below), so check that you actually qualify before you count on it.