real wall vs rotating region

Good day

For the model shown below, I am going to use the rotating region (sliding) option. So then I need to create a rotating region that covers the rotating impeller.

Okay so my question is do I make the rotating region to cover the whole impeller, or do I make it to just cover the blades of the impeller and then put in a boundary condition; real wall with motion rotating, for the impeller walls and hub and other components that also rotating with it.

So I need to know when and why do we use real wall with motion, can't i just (for this model) make the rotating region to cover everything that rotates (whole Impeller)

Thanks in advance.