5 U.S.C. § 8431: Certain transfers to be treated as a separation

(a) For purposes of this subchapter, separation from Government employment includes a transfer from a position that is subject to one of the retirement systems described in subsection (b) to a position that is not subject to any of them. (b) The retirement systems described in this subsection are—(1) the retirement system under this chapter; (2) the retirement system under subchapter III of chapter 83; and (3) any other retirement system under which individuals may contribute to the Thrift Savings Fund through withholdings from pay.