CSG (Closed Subscriber Group) or OAM (Open Access Mode)
CSG is a set of users (UE's, Subscribers) which allowed for a specific cell (usually femto cell). The cell with CSG Indication set to be 'TRUE' is called CSG cell.
Non CSG Cell (Ordinary Cell) allows any UE to camp on as long as the UE has proper PLMN info and the cell is not barred, *but CSG Cell allows only UE's belonging to a specific CSG.*
This information of being an CSG cell is asked in SIB1 with two information elements (CSG-Indication and CSG-Identity) can be set as false, absent like way.
A Network can allocate a certain range of PCI (Physical Cell ID) to be reserved for CSG. This can be defined by SIB9.