To establish a connection, your server and the remote device both send an XID.
The exchange identifier is defined in the controller description for your server. SNA hosts that use parallel connections require the XIDs. Your server conducts an initial poll with NULL XIDs during the connection process, to determine whether the remote device is active. If the device responds to the poll, your server and the remote device exchange XIDs and establish the connection.
The controller description for an SNA host names the XID value as the local exchange identifier. The APPC controller description names the XID value as the exchange identifier. Other controller types, such as CTRL, FNC, and RWS, also use the exchange identifier.