Which type of report is characterized by being under 90 days of service?

The correct answer is that the type of report characterized by being under 90 days of service is the NOB, or Navy Officer Billet. This report typically applies to officers who have been in their position for a short duration, specifically when they have served less than 90 days. The NOB is utilized to document performance in a way that acknowledges the limited time the officer has had to demonstrate their capabilities in the role.

Understanding the other report types provides context. The Letter of Extension is used for extending a period of service or duties, rather than assessing immediate performance. Concurrent reports, while they relate to Service members' assignments, often apply to members serving in multiple roles or aboard different units at the same time, which does not specifically pertain to the length of service in one position. FITREP, or Fitness Report, assesses an officer's performance but typically applies when they have been in their role for a longer period, generally over 90 days. Thus, the NOB is uniquely suited for situations involving brief service durations.