Belisaro (or legates) were high ranking officers in the Romulan Star Empire's Tal Diann. During times of war when Star Command established Tellas Vornai (field hierarchies) belisaros of the Tal Diann are given command of the unified Romulan Star Navy so not to upset the political balance by appointing existing flag officers. They commanded the forces in the Praetor's name.
During the Dominion War Ar'nal Velel was appointed by the Praetor as Ar'nal Belisarus, prime legate the Star Navy. (The Dominion War Sourcebook: The Fires of Armageddon)