What distinguishes a Rear Admiral (upper half) from a Rear Admiral (lower half)?

The distinction between a Rear Admiral (upper half) and a Rear Admiral (lower half) primarily lies in the insignia worn by the officer. The insignia for a Rear Admiral (upper half) features two stars, whereas a Rear Admiral (lower half) has only one star. This difference in the number of stars is a clear visual representation of their rank and authority within the Navy.

While stripes on the uniform may indicate rank to some extent, the specific insignia detailing with the stars is the definitive factor that differentiates between the two ranks. The color of the uniform does not vary between these ranks, and length of service does not directly affect an officer's rank designation in this context. Therefore, the presence of one or two stars is the key characteristic that delineates the two.