Sanctuary is a planet.
Starship Troopers novel
Sanctuary is a planet in Federation space. It is a human colony. Sanctuary has very little radiation, so although it is very similar to Earth in most ways it has very little life on it. Human colonists have brought their own plants and materials to the planet which quickly overran the native wildlife. The population largely supplies the Federation's military, based at Sanctuary and in orbit.
The Federal Military uses Sanctuary as a second major military base to keep its fleets safe from enemy attacks since the Arachnids did not know of its location.
Starship Troopers 3: Marauder
Sanctuary is hidden planet within the United Citizen Federation with at least one large ocean.
In orbit over Sanctuary are the Federal Fleet Headquarters which maintain the Fleet and consist of several orbital stations. Its location is top secret, as if the Bugs were to discover it, they would be able to destroy it and have free access deep into the Federation and Earth. Only a handful of Captains, including Lola Beck, know Sanctuary's coordinates.
During the Second Bug War, Dix Hauser takes Johnny Rico to Sanctuary's 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 carried seven Marauder Mech Suits. Rico and his troopers were then enlisted into the Marauder Project as the ship left the system.