From the Background: "The Department of Defense (DOD) operates a Military Health System (MHS) that delivers certain health entitlements under Chapter 55 of Title 10, U.S. Code. The Defense Health Agency (DHA)--a component of the MHS--administers the TRICARE program, which offers health care services to approximately 9.5 million beneficiaries, composed of military personnel, retirees, and their families. [...] This report answers frequently asked questions about TRICARE health plan options tailored for certain reservists, retired reservists, and their families (i.e., TRICARE Reserve Select and TRICARE Retired Reserve) and certain statutory prohibitions that limit their participation in the plans."
CRS Report for Congress, R45968
Congressional Research Service: https://crsreports.congress.gov/