SYSUAF.DAT is the user authorization file that holds user records on an OpenVMS system. By default, it is located in SYS$SYSTEM. A logical name, SYSUAF, can be defined to point to its current location.
Setting the UAFALTERNATE system parameter to 1 at boot defines the SYSUAF logical name to SYSUAFALT which causes the system to use SYS$SYSTEM:SYSUAFALT.DAT as the authorization file. If the file does not exist, the system lets you in with any password and displays an error message. If the file exists, the usernames and passwords from that file will be used.