$function vs $$function

Answers

Macro is $$$. Anyway, a little excerpt from documentation  

$
 

Dollar sign (36): Intrinsic (system) function prefix: $name(parameters).

Special variable prefix: $name.

$Znnn (a name beginning with $Z) can be a user-defined function or special variable defined using %ZLANG language extension library. It can also be an intrinsic (InterSystems supplied) function or special variable.

Regular expression end of string anchor; for example, (USA)$.

In ZBREAK debugging, a single-step breakpoint.

As first character at terminal prompt, load interactive subshell.

$$
 

Double dollar sign: Extrinsic (user-written) function prefix: $$name(parameters). $$ is returned by $STACK when context was established by an extrinsic function reference.

Prefix to a routine name to directly invoke that routine.

$$$ Triple dollar sign: Macro invocation prefix.