Final answer:
An NOB evaluation is designated for periods not exceeding 90 days, to account for situations where a service member is not observed performing duties due to short assignments or temporary status.
Step-by-step explanation:
An NOB evaluation (Not Observed Block) is submitted for short periods of duty or temporary duty not to exceed 90 days.
This type of evaluation acknowledges that the rated member has not been observed performing duties for a sufficient period of time, which is why it is limited to short duty assignments or temporary status.