MSS is limited by IBM mainframe compatibility to 8-character usernames. However, it is now possible to use usernames and passwords of up to 100 characters in Enterprise Server, with some restrictions and additional considerations. See Long Usernames and Passwords for more information.
Restrictions still apply to JES and JCL support. Any name referenced in JCL must be uppercase (for example, USER=XXX), as you cannot use a lower-case or mixed-case username in JCL. You can have a user with a lower- or mixed-case name, including names longer than eight characters, to submit jobs. The USER parameter of JCL job cards are still restricted to eight characters.