The three current options are each defined separately in section 1072. TRICARE Prime, at 1072(14), is "the managed care option of the TRICARE program", meaning an assigned primary care manager, a network, and referrals for specialty care. TRICARE Select, at 1072(12), is "the self-managed, preferred-provider network option under the TRICARE program established by section 1075", and section 1075(a)(2) adds that beneficiaries in Select "will not have restrictions on the freedom of choice of the beneficiary with respect to health care providers". The trade between the two is the familiar one: Prime charges less and constrains where care is received; Select costs more per service and does not. Two further definitions at 1072(11) and (15), TRICARE Extra and TRICARE Standard, describe options made available before January 1, 2018 and are legacy vocabulary that still turns up in older material.
Section 1075(b)(1) sorts Select enrollees into cost-sharing categories, and the way it does so previews the program's most consequential rule. There is an "active-duty family member" category and a "retired" category, and the retired category is expressly defined to exclude "Medicare-eligible beneficiaries described in subsection (d)(2)" of section 1086. Those beneficiaries are not left out of coverage; they are the population TRICARE For Life exists for.
The Medicare interaction is the single most useful thing on this page. Section 1086(d)(1) states flatly that "A person who is entitled to hospital insurance benefits under part A of title XVIII of the Social Security Act is not eligible for health benefits under this section." Section 1086 is the provision that covers retirees and their families, so on its own that sentence would end TRICARE coverage the moment a retiree became entitled to Medicare Part A. Paragraph (d)(2) supplies the exception, and the exception is conditional: the prohibition does not apply to a person who "is enrolled in the supplementary medical insurance program under part B", with an additional requirement for people under 65 that their Part A entitlement arise through the disability or end-stage renal disease routes. Enrolling in Medicare Part B, and paying its premium, is therefore the price of keeping TRICARE as a Medicare-eligible retiree. This is exactly the decision the Medicare late enrollment penalty punishes people for getting wrong, and the two consequences compound: declining Part B both ends the TRICARE benefit under section 1086 and starts a penalty clock.
What TRICARE For Life then does is pay second. Section 1086(d)(3)(A) addresses care "for which payment may be made under medicare and a plan contracted for under subsection (a)", which is to say care both programs cover, and directs the plan to pick up the beneficiary's out-of-pocket cost after Medicare and any other third-party payers. Subparagraph (B) then caps that: the plan may not pay more for the care than "the total amount that would be paid under the plan if payment for that care were made solely under the plan". The practical reading is that where Medicare and TRICARE both cover a service, the beneficiary usually has little or nothing left to pay, and where only one of them covers it, the ordinary rules of that one program apply.
Two boundaries are worth drawing. TRICARE is not VA health care. TRICARE is a Department of Defense program under Title 10 for service members, retirees and their families; the Veterans Health Administration is a separate system under Title 38 for veterans, with its own eligibility rules and its own facilities. Some people qualify for both. And TRICARE is not Medicare: it does not replace Part A or Part B, and for a Medicare-eligible retiree it operates alongside them rather than instead of them.
Costs move. Enrollment fees, annual deductibles, catastrophic caps and pharmacy copays differ by plan option, by whether the sponsor's service began before or after January 1, 2018, and by whether the beneficiary is an active-duty family member or a retiree, and they are adjusted annually. The dollar figures written into section 1086(b) belong to the older structure and are not what a beneficiary pays today. Any specific amount should be read from the program's current published schedule at the time it matters.