What is the roll of the IaaS component of the vCAC architecture?

I'm getting the lay of the land in vCAC world.  I'm a bit confused as to the purpose of the IaaS bit.  I thought that I understood the vCO to be the engine that does all of the automation.  However, it sounds like the IaaS appliance is doing a bunch of the automation.

Can someone please clarify the purpose and function of the IaaS appliance, and differentiate from the vCO where fitting?

Thanks!