The Galactic Empire: An Empire Reborn
Imperial Officers' Manual
Major Revision: 2.0 – 2008.10.31
An adaptation of the pre–Endor document and policies.
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... A Rear Admiral's Rank Insignia ...

The rank of Rear Admiral (RA) is an "intermediate rank"... It is conferred to honour exceptional Commodores, after sufficiently long & dedicated service to the Empire.

A Rear Admiral does, of course, possess seniority, and – whereas the rank of Commodore is classified as a lower variant of admiral – it is at this rank that one achieves full & true admiralty.