5 CFR 842.1004 - Evidence.
(a) The agency head's determination under § 842.1003(a) that a position is a primary position must be based solely on the official position description of the position in question, and any other official description of duties and qualifications. The official documentation for the position must establish that it satisfies the requirements defined in § 842.1002.
(c) If an employee is in a position not subject to the one-half percent higher withholding rate of 5 U.S.C. 8422(a)(3), and the employee does not, within 6 months of entering the position formally and in writing seek a determination from the employing agency that his or her service is properly covered by the higher withholding rate, the agency head's determination that the service was not so covered at the time of the service is presumed to be correct. This presumption may be rebutted by a preponderance of the evidence that the employee was unaware of his or her status or was prevented by cause beyond his or her control from requesting that the official status be changed at the time the service was performed.