IRIS and Caché / Ensemble offer the possibility to add Languages Extension as SystemFunctions,  SystemVariables,  SystemCommands

The related documentation tells you where to add the system-wide extension. Parameter passing is not covered explicitly.
In principal, it is always a call for a subroutine. And at the end, you terminate with QUIT  returning a value or not.

SystemFunctions
You call them as any other standard system function with all required parameters in parenthesis.
   eg $ZZR(47,11)  and it will be processed. Just mentioned for completeness.

SystemVariables
There are 2 use cases: requesting a value  WRITE $ZZR   or  setting a value   SET $ZZR=17 
The related common function has to detect what is going on. Like this:

SystemCommands
This is a little bit tricky as a distinction between multiple arguments and multiple parameters is required.
Multiple arguments are separated by a comma "," :   ZZR arg1,arg2,..

But if you require more than a single value as parameters
they have to be separated by colons ":"    :   ZZR par11:par12:par13, par21::par23, ::par33, par41, :par52
                   or just nothing   ZZR

The code should be prepared to handle missing parameters.
It is the decision of your code if the last parameter is interpreted as a timeout or anything else.
It is typically ended with QUIT without returning any value.