32 CFR 199.17 (c)(1)
Active duty members. Active duty members are required to enroll in Prime when it is offered. Active duty members shall have first priority for enrollment in Prime. Because active duty members are not CHAMPUS eligible, when active duty members obtain care from civilian providers outside the military medical treatment facility, the supplemental care program and its requirements (including § 199.16) will apply.
32 CFR 199.16 (d)(2)
Preauthorization by the Uniformed Services of each service is required for the supplemental care program except for services in cases of medical emergency (for which the definition in Sec. 199.2 shall apply) or in cases governed by the TRICARE Prime Remote program for active duty service members set forth in paragraph (e) of this section. It is the responsibility of the active duty members to obtain preauthorization for each service. With respect to each emergency inpatient admission, after such time as the emergency condition is addressed, authorization for any proposed continued stay must be obtained within two working days of admission.
32 CFR 199.2
Medical emergency. The sudden and unexpected onset of a medical condition or the acute exacerbation of a chronic condition that is threatening to life, limb, or sight, and requires immediate medical treatment or which manifests painful symptomatology requiring immediate palliative efforts to alleviate suffering. Medical emergencies include heart attacks, cardiovascular accidents, poisoning, convulsions, kidney stones, and such other acute medical conditions as may be determined to be medical emergencies by the Director, OCHAMPUS, or a designee. In the case of a pregnancy, a medical emergency must involve a sudden and unexpected medical complication that puts the mother, the baby, or both, at risk. Pain would not, however, qualify a maternity case as an emergency, nor would incipient birth after the 34th week of gestation, unless an otherwise qualifying medical condition is present. Examples of medical emergencies related to pregnancy or delivery are hemorrhage, ruptured membrane with prolapsed cord, placenta previa, abruptio placenta, presence of shock or unconsciousness, suspected heart attack or stroke, or trauma (such as injuries received in an automobile accident).