5 USC 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

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(3) any other retirement system under which individuals may contribute to the Thrift Savings Fund through withholdings from pay.