From [Redacted]:
The Administratum codifies Knight Worlds as planets autonomous from the Imperium and host to one or more Knight Houses. These ancient institutions consist of noble bloodlines pre-dating the Imperium by many thousands of years, most having settled their home world in the far distant and all but forgotten Dark Age of Technology. The scions of the Knight Worlds are oath-sworn to protect their subjects from the numerous threats that assail them, and they do so mounted in towering suits of Knight Armours, smaller than, but distantly akin to, the mighty god-engines of the Collegia Titanicus.