The Fleet Base, as its name suggests, is the base of the Fleet.
Starship Troopers novel
Rodger Young went back to the Fleet Base for capsules for several times.
Starship Troopers 3: Marauder
Sanctuary Federal Fleet Heaquarters is the home base of Fleet during the Second Bug War, which consists of several stations in orbit to the planet Sanctuary (or at least near to it). Its location is Top Secret as if the Bugs were to discover the location they would be able to severly damage the Federal Fleet. Thus they would have free access deep into the federation and unrestricted access to Earth. Only a few handful of Captains, including Lola Beck, know the coordinates of its location.
During the Second Bug War, Dix Hauser took John Rico to Sanctuary Federal Fleet Headquarters along with a surviving squad of his Troopers from the Battle of Roku San. They were then transported to the Athena class starship Shiloh, which was containing seven M11 Babar Marauders. Rico and his troopers were then enlisted into the Marauder Project as the ship left the system.