System variables are used to communicate special information between the dialog and the dialog manager (ISPF). System variable names are reserved for use by the system. They begin with the letter "Z". Therefore, avoid names that begin with "Z" when choosing dialog variable names.

The types of system variables are input, output, non-modifiable, and input-output. Their type depends on their usage.

To access and update system variables, use variable services according to which pool the variables are in. System variables in the function pool can be accessed and updated directly from a command procedure. Those in the shared or profile pools can be accessed by using the VGET service, and updated by using the VPUT service.

A program function can access and update system variables in the function pool using the VDEFINE service. Dialog variables can be accessed by using the VCOPY service and updated by using the VREPLACE service.

The system variables in the shared or profile pools can be accessed by using the VCOPY service. They can be updated by first updating the variable in the function pool by using the VDEFINE or VREPLACE service and then moving that value to the shared or profile pool by using the VPUT service.