Tupile was a secretly held region within the known universe. While peripherally considered a single planet (the "sanctuary planet"), it was actually a group of several planets.
The exact location of Tupile was known only to the Spacing Guild, and was not disclosed to other groups or organizations.
During the time of the Faufreluches under House Corrino, Tupile was used chiefly as a location for defeated Great Houses of the Imperium for sanctuary when exiled. This occurred if the House in question either defied the Emperor or was defeated in a War of Assassins or in an act of Kanly by a rival Great House.
While Tupile's purpose may have remained as such during the early parts of the Atreides Empire, during and after the rule of Leto Atreides II, there was evidence to suggest that some houses (such as the remnants of House Harkonnen) were not exiled to Tupile. Thus the area lost some of its mystique and stigma.