The term 'Periods of Duty' can be summarized as a series of short assignments or 'stints' (6 letters). These stints are typically short-term commitments that require a person to perform a certain task or job for a specific amount of time. This could include anything from working a shift at a restaurant to volunteering for a charity event. The length of the assignment can vary, but it is usually no longer than a few weeks or months.