Defense Enrollment Eligibility Reporting System (DEERS) applies to Active Duty service members and their dependents (TRICARE only). Active Duty service members MUST be registered in DEERS to receive TRICARE benefits.
DEERS is a database of information on uniformed service members (sponsors), U.S.-sponsored foreign military, DoD and uniformed service civilians, other personnel as directed by the DoD, and their family members.
Resource: Defense Enrollment Eligibility Reporting System | TRICARE