The Prime Fleet of the Golden Empire was the centerpiece of the Glorious Armada of the Golden Empire and the only permanent Royal Navy fleet. It was the personal command of the Prime Legate and, as it included the Sith Star, was also a sort of mobile capital for the entire Empire.
Shortly after the foundation of the Golden Empire in 87 ABY, Rin Sakaros fought a duel with her father Tak. Having defeated him, she took his fleet for herself and set out to forge an Empire in reality as well as on paper. During this period, the Ta'asyak Za`vek was usually just called "the fleet", as there was only one.
Expanding with the acquisition of Iscandar and its small navy in 95 ABY, the entire Royal Navy still numbered fewer than fifty capital ships at the onset of the Great Liberation. During that war, several Tetrarchy ships were captured and converted, while others were surrendered upon the capture or defection of Tetrarchy worlds. Though the Navy was often kept together, when it was split for any purpose, the fleet containing the Sith Star was considered the "prime fleet" of the Empire. This designation came to be the official name of the fleet itself.
After the war ended, the Prime Fleet was officially designated the first fleet of the Empire, both symbolically and militarily. With the Empire's unique Legate system of command, only the Prime Fleet existed permanently. It continued to fight on the front lines of the Empire's wars, including the Conquest of the Chiss Ascendancy and the Vagaari War. It avoided most of the Nightmare War only to lead the climactic Battle of Dolomir in 149 ABY.
The Sith Star was the only ship of the line which was permanently attached to the Prime Fleet. Although many capital ships spent years or even decades routinely assigned to the Prime Fleet, they could be rotated out to the command of Legates as needed or reassigned at will.
However, once sufficient ships were available, the Prime Fleet never contained fewer than fifty capital ships. The Sith Star usually had at least a pair of Imperial Star Destroyers or Devastator cruisers (or both) as escorts, and numerous frigates, corvettes, cruisers, and other Star Destroyers made up the remainder of the fleet.