AmigaOS Manual: ARexx Special Variables

There are three special variables that ARexx sets automatically during execution of a script:

RC	RC is set to return code from any executed host command. If an ERROR or SYNTAX interrupt occures, RC is set to the error code that triggered the interrupt. The NOVALUE and HALT interrupts have no effect on RC.
RESULT	RESULT is set by a RETURN instruction in a subroutine that has been called. If the RETURN instruction has no expression on it, then RESULT becomes uninitialized.
SIGL	SIGL contains the line number of the last instruction that caused a jump to a label.

None of these variables have an initial value. They may be altered by the user, but ARexx will continue to set them automatically when appropriate. The PROCEDURE and DROP instructions also affect these variables.