Just as traditional APL interpreters have system variables and system functions (whose names all begin with the Ś character), system methods are pre-defined methods (also with names beginning with Ś) which apply to internal user-defined object classes, and in most cases to external classes as well.
You can call a system method exactly as you would call an ordinary method of a class, either using dot notation (for example MyPoint.ŚCLASSNAME), or (within a user-defined class method) by simply using the system method's name such as ŚCLASSNAME (equivalent to ŚTHIS.ŚCLASSNAME).
System methods can be niladic, monadic or dyadic.
See the reference section on System Methods for a full list.