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I am trying to wrap my head around this. If I grab a knight in my SW army I need to pay 3 CP to put it in a detachment, that I do not get back.

"If this Detachment is a Super-heavy Auxiliary Detachment that contains one FREEBLADE unit, until the end of the battle, that unit gains the AGENT OF THE IMPERIUM keyword. Only one FREEBLADE unit in your army can have this keyword."

However, if I grab an Inquisitor they do not take up a slot in a detachment and is thus free?

Yes, a knight in a shaux detachment costs you 3CP.