When Black Industries published Dark Heresy, they openly requested fans to submit descriptions of planets. Various of the submissions they received were edited and used to expand the newly-created Calixis Sector, becoming an official part of the game's setting. The core rulebook of course had limited space, so the full descriptions for all the planets that Black Industries accepted were published on the company's website, forming a freely-accessible gazetteer of the Calixis Sector.
Following Games Workshop's closure of the Black Industries imprint, Fantasy Flight Games acquired the licence to publish role-playing games set in the Warhammer Fantasy and Warhammer 40,000 universes. They retained an on-line gazetteer for the Calixis Sector, but in a cut-down form that detailed only the major worlds.
But fans had saved copies of the full original content and compiled it into a PDF titled "The Calixis Sector Encyclopedia". This PDF can still be found, circulated by fans on various internet forums dedicated to discussing Dark Heresy and related games.