If you are using the default SYSIN configuration, your have a MUF Startup Option called "MUF" that is set to * (asterisk). If you are using this recommended default, it will pick up the MUF name from the TARGET_MUF_LIST=() setting in your DBSIDPR settings.
Also, we recommend that you name the CXX similarly to the MUF name. This recommendation is for a name of one to seven characters, which leaves an extra character for a MUF name suffix in the case of a Shadow MUF configuration. If you are upgrading an existing MUF, we recommend you keep the same CXX name as in your current MUF. Note that if you are using the Simplify feature of version 14.0, you will need to specify the CXXNAME= value in your DBSIDPR settings.
For further information about the MUF Startup Options or the DBSIDPR settings, please see the CA Datacom/DB Database and System Administration Guide, in the section, "Using the Multi-User Facility" and the subsections "Modifying MUF Startup Options" and "Modifying DBSIDPR Parameters."
As always, please contact CA Technologies support for CA Datacom if you have further questions.