Orders over 30 days — one page

Recut from TRICARE.mil: drill, AT, and orders of 30 days or less stay inactive. Buy TRS. Line of duty care is the injury door. Family is not on LOD.
More than 30 days: you are not TRS. Personnel posts DEERS. Active-duty TRICARE starts on day one of the orders for you and DEERS-registered family.
Deactivating is a qualifying life event — 90 days to change the plan.
Contingency / 12304b / national emergency: TAMP 180 days after. Then buy TRS again if you still drill. Other activations: no TAMP.
IRR and FEHB-eligible members do not buy TRS. Official door only.

Official door: TRICARE activation / deactivation

/start/reserve

Independent educational one-pager. Not Army, VA, or DoD. If a rate or a form changes, trust the .gov / .mil page. VA and TRICARE never charge a fee to apply.