The manual talks about taking direct command of a corps. I haven't figured out how to do this yet.
I have had another look at this (section 4.5 of book 1) and I think there is a bit of confusion here. One way to think of this is to consider who issues the orders and who carries them out.
I think that being a corps commander involves simply commanding regiments to detach and perform orders. The following orders can be issued by the player as the corps commander: Move, Reconnaissance, Attack, Bombard, Support. In this context, the corps commander issues the orders and the colonel carries them out.
This differs from regimental level orders (issued by the colonel) which as far as the player is concerned mainly involves deciding formation: Column of sections, squads or divisions, and Line for infantry (other formations for cavalry). The manual also says the the colonel can halt and resume the regiments movement as well as detach skirmishers, but can the player do that personally? In this context, the colonel issues the orders and the battalion commanders carry them out.
EDIT: Sort of similar to the questions asked above, does a detached regiment suffer any morale penalties for being detached? Essentially, is saying a regiment is detached just a way of saying that the player has direct control over it and it is no longer controlled by Regimental AI; or does a detached regiment actually see itself as removed from the corps command structure and support and therefore suffer possible morale penalties?