During a Start of New User Session request or a User Login Request or an RPCSEC_GSS User Session Request (see the Note that follows), the exit can obtain a User Exit Block. The exit returns a word to the server. This word is referred to as ‘the address of the UXB’, but the system initialization exit might store any value in the word. The address of the UXB is returned to the server in the parameter list (depends on which request comes first), and is passed back to the installation-wide exits on each subsequent call related to this combination of machine and user IDs.

The UXB can contain an area to save the user data needed for this session. The usage of this block is determined by the exit. The exit is responsible for obtaining, and freeing access to these storage areas. This block is used by the login installation-wide exit and file security installation-wide exit. The format of the UXB is entirely controlled by the login and file security installation-wide exits.