Ok, so a "formation detachment" isn't spelled out as anything specific in the rules. What it is in BS is a detachment made up or one or many formations. In the case of the Renegade Knights, there are no formations for them, and a grand total of one model for the entire faction right now. They can't be taken in a CAD, they can't be Allied Detachment because there's no LoW slot in an Allied Detachment. Instead, it is listed in BS as the "Chaos RK Forsworn Knight Detachment."