5 CFR 1601.33: Acknowledgment of risk.

(a) Uniformed services participants who first entered service prior to January 1, 2018, and who have not elected to be covered by BRS and civilian participants who enrolled prior to September 5, 2015, must execute an acknowledgement of risk in order to invest in a TSP core fund other than the G Fund. If a required acknowledgment of risk has not been executed, no transactions involving the fund(s) for which the acknowledgment is required will be accepted.

(b) The acknowledgment of risk may be executed in association with an investment election, a fund reallocation, or a fund transfer in the form and manner prescribed by the TSP record keeper.