5 CFR §846.203

1. (a)Employing offices must distribute the election forms provided by OPM to each eligible individual, including all individuals in a nonduty status.
2. (b)An employing office must obtain documentation of the individual's receipt of the election form specified in paragraph (a) of this section and retain the documentation permanently in the individual's official personnel folder (or the equivalent). Acceptable documentation includes—
   1. (1)A statement of receipt signed by the individual, or
   2. (2)A signed postal return receipt showing that the election form was received at the individual's address.