20 C.F.R. § 204.5
For the purpose of crediting deemed service months as provided in § 210.3(b) of this chapter, an individual must have maintained an employment relation to one or more employers in the month or months to be deemed. For that purpose an employment relation exists with respect to any month in which an individual, although not in the active service of an employer, is on furlough subject to recall by an employer, is on a bona fide leave of absence, has not been retired or discharged but was by reason of continuous disability unable to return to service, or was not in active service because of a discharge later determined to be wrongful. However, an employment relation with respect to an employer ceases after an individual has resigned or relinquished his or her rights to return to the service of that employer or after the individual becomes entitled to receive an annuity under the Railroad Retirement Act.
20 C.F.R. §204.5