Much of the overall behavior of the system is dictated by what are known as system values. System values aren't objects, like programs and files, because they cannot be created or deleted; they can only be changed and displayed. Approximately 150 system values are available in V5R3. Some of them control system performance, others control security, and others provide defaults to common settings. A complete discussion of system values is beyond the scope of this book, which concentrates only on the most important ones.
Many commands accept *SYSVAL as a valid value in certain parameters. *SYSVAL is a reference to a system value. For example, in the Create User Profile (CRTUSRPRF) command, one of the parameters is ATNPGM, which determines what program to run when the user presses the Attention key (ATTN). ATNPGM accepts *SYSVAL. If you choose *SYSVAL, the user profile points to the appropriate system value, which is named QATNPGM and contains the name of the program required in the CRTUSRPRF's ATNPGM parameter.
The advantage to using *SYSVAL is obvious. If you want to give most of your users the same attention-key program, you would be better off plugging its name into system value QATNPGM and using ATNPGM(*SYSVAL) in the user profiles. That way, you can change all those users with a single keystroke by only changing the system value.