18 (d) A person who is otherwise qualified, but who has a physical defect that the Secretary concerned determines will not interfere with the performance of the duties to which that person may be assigned, may be enlisted as a Keserve of any armed force under the jurisdiction of that Secretary. § 511. Reserve components: terms (a) Except as otherwise prescribed by law, enlistments as Keserves are for terms prescribed by the Secretary concerned. However, an enlistment that is in effect at the beginning of a war or of a national emergency declared by Congress, or entered into during such a war or emergency, and that would otherwise expire, continues in effect until the expiration of six months after the end of that war or emergency, whichever is later, unless sooner terminated by the Secretary concerned. (b) I n time of war or of national emergency declared by Congress the term of service of an enlisted member transferred to a reserve component according to law, that would otherwise expire, continues until the expiration of six months after the end of that war or emergency, whichever is later, unless sooner terminated by the Secretary concerned. § 512. Reserve components: transfers (a) A person who would otherwise be required to be transferred to a reserve component under section 651 of this title or under sections 451-473 of title 50, appendix, is entitled, if he is qualified and accepted, to be enlisted in any armed force that he chooses and to participate in the programs authorized for that armed force. However, unless the two Secretaries concerned consent, he may not be enlisted as a Reserve of an armed force other than that from which he is transferred. All periods of his participation shall be credited against the total period of service required of him under section 651 of this title or under sections 451-473 of title 50, appendix. However, no period may be credited more than once. (b) A person covered by subsection (a) shall perform the rest of his required term of service in the armed force in which he is so enlisted or in any other armed force in which he is later enlisted or appointed. (c) This section does not change any term of service under an appointment, enlistment, or agreement, including an agreement made before or at the time when the member entered upon a program authorized by an armed force. § 513. Reserve components: promotions The Secretary concerned shall establish an adequate and equitable system for promoting enlisted members of the reserve components under his jurisdiction who are in an active status. So far as practicable, each promotion system shall be similar to that for members of the regular component of the armed force concerjied. Each promotion system shall provide for forced attrition to the extent necessary to maintain vigorous reserve forces, encourage necessary leadership, and provide a steady flow of promotions.