CSG and CUG these are two concepts which are coined to increase subscriber base for the operator, where CUG is completely marketing gimmick where as CSG is Marketing + resource saving.
In summary a closed subscriber group (CSG) is a limited set of users with connectivity access to a femtocell or BTS/NodeB. It is mostly (in practicle sense only femto) used in femoto.
Use Case: When a Femto is configured in CUG mode, then it will accept LAU (Location Area Update) only from a subscriber who is in the ACL (Access Control List) of the femto, the other subscriber, named as Public User, who is not in the ACL will get a reject.
