20 CFR §1002.41

USERRA rights are not diminished because an employee holds a temporary, part-time, probationary, or seasonal employment position. However, an employer is not required to reemploy an employee if the employment he or she left to serve in the uniformed services was for a brief, nonrecurrent period and there is no reasonable expectation that the employment would have continued indefinitely or for a significant period. The employer bears the burden of proving this affirmative defense.