System symbols are defined in IBM publication SA22-7592 z/OS MVS Initialization and Tuning Reference. You can use dynamic system symbols, system-defined static system symbols and installation-defined static system symbols in your symbol dictionary and MFX control statements.
The string you specify as a symbol string can consist of a mixture of characters and all three types of system symbols. You can also build strings using concatenation and substrings. If you need to specify an apostrophe in the string, use two single apostrophes.
System symbols must be specified in uppercase.
When system symbols are encountered in a symbol dictionary, they are converted to the appropriate character string and then processed as any other symbol.