Problems of HCM Step "Define Target Areas for Users"

Dear experts,

ERP Initial Package for HCM Personnel Dev. PD & PA

-> Configuration and Selection -> Customizing -> Define Target Areas for Users.   I don't understand the usage of this step.

(1) Is the value range used to select the data in the sender system ?

(2) Do I need to define one target area for each object type?

(3) If don't maintain all the object types, what will happen to the object types not maintianed here?

(4) When can I find a the list of all the object types?

Thanks,

Claire