APL system functions implement a wide variety of system-related or utility features. They are built-in to the APL interpreter, but often call out to the operating system to perform some function (such as reading from a database, or fetching the current date and time). They have names of one or more characters and start with a Œ (ŒBOX ŒTS ŒWS etc).
System functions can be niladic, monadic or dyadic.
See the reference section on System Functions and Variables for a full list.