The Called Subscriber Identification (CSID) is a string that identifies a fax as a recipient. It's sent by a fax machine when receiving a fax from a transmitting machine. The CSID is displayed on the sending fax machine, this helps to confirm that the fax is being sent to the correct recipient. This string is usually a combination of the fax or telephone number and the name of the sender and is often the same as the Transmitting Subscriber Identification (TSID).