On entering the security exit, Register 1 points to a list of ten full words, as shown in this diagram.

The contents of the parameter list are as follows:
Is a full word containing the address of function code F'12'.
Is set to zero.
Is the address of 8-character field, blank-padded, containing the user ID logging off.
Is the address of three full words containing the user session correlator (if any) and the user token (UTOKEN) address, set by the exit when this user ID logged on, or zero.
Are set to zero.
The exit must return a completion code in register 15 on return. The only valid completion code is F'0'.
Note: It is possible for the exit to receive a logoff call for a user who has never successfully logged on. This might occur when a user abandons their logon attempt due to a forgotten password. The exit must be written to accept such calls and to ignore them.
Your system services generate a logoff call during DISCONNECT processing on behalf of an internally created environment. Because there was no logon request call for this environment, there is no session correlator created, and it is, therefore, null.
When a users logs back on to reconnect, a logon request call is made to the security exit. A new session correlator is created at this time. When the user selects a RECONNECT or CANCEL of all disconnected environments, a logoff request call is made to the security exit with the original user session correlator. The reconnected or new session continues with a new user session correlator that is presented to the security exit upon that session's logoff.
A zero user session correlator on a logoff request call to the security exit is valid for a DISCONNECT request only.