Closed subscriber group
A closed subscriber group (CSG) is a 3GPP term used to describe a limited set of users with connectivity access to a femtocell. When a femtocell is configured in CSG mode, only those users included in the femtocell's access control list are allowed to use the femtocell resources. On the other hand, a femtocell can be also configured in Open Access mode, in which any user is allowed access to the femtocell.
A closed access femto will accept LAU (Location Are Update) only from the subscriber which are in the ACL (Access Control List) of the femto the other subscriber which is not in the ACL list (called as Public User) will get reject with a "roaming not allowed" cause 13 or 15.