Internal CFD analyses can be conducted at zone, building block and building levels. Calculations can also be conducted for single zones that span several blocks by connecting them with holes and using the Merge zones connected by holes model option setting.
External analyses can only be conducted at the site level.
The level you are at when you create the CFD Analysis defines both the type of the CFD analysis (internal or external) and also the extent of the CFD domain. If you are at building level then the domain is the whole building, if you are at block level the domain will be the block and if you are at zone level the domain is the current zone.