Anytimw when you enter a CSG coverage area or leave a CSG coverage area, UE will inform eNB about this cell id through this message. eNB will take this into account while prioritizing for handover. Since you have higher priority for CSG cells, eNB needs to know about the visible CSG cells and will handover you to the best ranked cell.
In Idle mode, you don't need to send this to eNB and UE should locally use this information for Cell-Reselection to the best ranked CSG cell.
