(a) All service at the required or higher grade (or equivalent) in positions to which appointed in the Federal civilian service is creditable towards the time periods required by § 300.604 of this part, except as provided in paragraph (c) of this section. Creditable service includes competitive and excepted service in positions under the General Schedule and other pay systems, including employment with a nonappropriated fund instrumentality. Service while on detail is credited at the grade of the