System variables are a special class of predefined variables available to all program units. Their names always begin with the exclamation mark character (!). System variables are used to set the options for plotting, to set various internal modes, to return error status, etc.
System variables have a predefined type and structure that cannot be changed. When an expression is stored into a system variable, it is converted to the variable type, if necessary and possible. Certain system variables are read only, and their values cannot be changed. The user can define new system variables with the DEFSYSV procedure.
System variables are discussed in System Variables.