Besides your general medical care and related diagnostics and testing, there are over 200 specific items that may be covered by TRICARE, depending on your level of coverage.
Visit https://tricare.mil/CoveredServices/IsItCovered.aspx for a searchable, complete list of covered treatments, tests, and medical equipment, as well as details on which plans will cover them.
In general, any treatment, service, or products that are not medically or psychologically necessary will not be covered by TRICARE. The list of excluded items includes things like homeopathic medicine, certain psychiatric treatments, elective and cosmetic procedures, gym memberships and weight loss counseling, massage, and nursing homes and assisted living facilities.
For a complete list of items excluded from all TRICARE plans, visit https://tricare.mil/CoveredServices/IsItCovered/Exclusions.aspx
If a provider accepts TRICARE, you are only responsible for paying your copayment (if applicable). If the provider is not paid for the remaining balance for any reason, you will not be responsible for it. It is therefore up to the provider to ensure that their treatment complies with TRICARE guidelines regarding coverage for specific treatments, not yours.
NOTE: There are many items that are not normally covered that will be covered if you are given prescription from a doctor for them, or, similarly, that will be covered if they are determined to be medically or psychologically necessary as part of a treatment that is covered.