20 C.F.R. § 332.6
Subject to the provisions of § 332.2(b) , a day shall not be considered as a day of unemployment or as a day of sickness with respect to an employee if no remuneration is payable or accrues to him solely because he is standing by for or laying over between regularly assigned trips or tours of duty. Only employees who hold regular assignments may be regarded as standing by for or laying over between regularly assigned trips or tours of duty. In determining whether an employee has a regular assignment, consideration shall be given to whether the trips or tours of duty have definite starting times; whether there are a definite number of trips or tours of duty, either periodically or for the whole duration of the assignment; and whether there is a definite route of each trip or definite duration of each tour of duty. An employee who is separated from a regular assignment shall not be regarded as standing by for or laying over between regularly assigned trips or tours of duty. An employee shall be deemed separated from a regular assignment when he is suspended or discharged from service or displaced by a senior employee or held out of service for investigation or discipline, or when his regular assignment is abolished or discontinued.
20 C.F.R. §332.6