... An Admiral's Rank Insignia ...
The promotion to the coveted rank of Admiral (AD) is one of the most sparingly given and sparsely available in all of The Galactic Empire...
The rank of full Admiral is above what any officer can reasonably hope to achieve in their Imperial career, and is -- for all practical intents & purposes -- the highest any officer can achieve. As perspective, even many of the Empire's full Division Commanders do not carry seniority equivalent to that of a full Admiral.
Within the pre-Endor Empire, an Admiral in standard Imperial Navy service commanded a Systems Force (a naval force assigned to a region containing dozens of inhabited star systems within a sector), and was often informally distinguished by the term Systems Admiral. This is no longer correct. Due to the thinning of Imperial resources, smaller fleets are now responsible for much larger regions of space, and even the command of a typical Vice Admiral has been increased to span the naval forces of an entire sector, or more.
Currently, a proven & longstanding Imperial Navy Fleet Commander stands a reasonable chance of being promoted to Admiral... Certain other High Command positions may also technically allow for it to be conferred upon holders of incredible longevity & dedication.
At a full Admiral's level, the amount of Imperial might that can be rallied by a single individual approaches staggering levels... And the prospect of ever crossing one is enough to make any line officer think twice. These are, after all, men who can make and break careers at a word.