The aggregation levels should only be configured in situations where you are collecting the same data at multiple levels of the hierarchy. This generally should never be the case, but it can happen in certain circumstances. Please see this old post for a good description.
Org unit group simply refer to a group of organisation units which are related in some way. For instance, you might have two groups “Public” and “Private”. Each facility could then be assigned to one of the groups, depending on its circumstances. An organisation unit group set consists of a number of different groups which are related. So in this case, the group set might be called “Ownership” and consist of two the groups “Public” and “Private”.
Hope this helps to clarify.
Regards,
Jason