Previously, this book discussed how to call another program as an external routine (Writing Subroutines and Functions). You can also call a program from another program explicitly with the EXEC command. Like an external routine, a program called explicitly or implicitly can return a value to the caller with the RETURN or EXIT instruction. Unlike an external routine, which passes a value to the special variable RESULT, the program that is called passes a value to the REXX special variable RC.