Calling a function in RAP is no different than in Object Pascal. As long as the function is in scope, you can call it.
Note: When calling a parent report's global function from within a subreport, you do not need to qualify the identifier with the parent report's name.
In other words, if your Main report has a global procedure, GlobalDoSomething, and you want to call it from within SubReportA's OnCreate event, you do not have to say Main.GlobalDoSomething. Merely calling GlobalDoSomething will suffice.