The regions of ANY type of group affect who can 'see' and be added to that group (or will be automatically added in the case of an Everyone Group).
A normal Item Access Group can only contain items from the systems to which the group applies. So a ‘Region A’ Item Access Group can only contain items from ‘Region A’ systems.
For Common Item Access Groups, although the region still controls the visibility of the group in exactly the same way, the items it contains can only ever be RRMS items. RRMS items have no home system and no home position (they are essentially unallocated iFobs). As these items would never be limited to coming from a particular system, you can always add ANY unallocated item to a single Common Item Access Group.
NOTE: That once an item is added to a Common Item Access Group, it cannot be added to any subsequent groups.
Users may expect that items that are physically bound to a department or building would be filtered to the region that the systems are in but that is not the case. The allocation of RRMS items into Common Item Access Groups is essentially inferring the systems which that item can be placed into - hence the 'Common Group' term. If an item is placed into a system outside those regions, it will be unrecognised and only removable by an Administrator.
To summarise:
An Item Access Group collects together similar iFobs/Items from one or more systems that can be granted to a number of users of those systems. Each item retains its individual access level
A Common Item Access Group collects iFobs/Items together and gives them all the same access level across all the visible systems/regions in the group. This can then be applied to multiple users of those systems