by a piece of data, you presumably mean a block of untyped memory? i would suggest that is not how most people view a union, and that your union has either one or two members, depending on how it is used.
Correct, there is one contiguous block of memory that can be interfaced in multiple manners. It is still the same block of data, in much the way EAX is just a portion of RAX; not an independent register.
