There are hundreds of regulations regarding auditions, callbacks,
work hours, pay scale, P&W, meals, travel, lodging. No way to
enlighten you on all the rules. But the SAG website can.
If you are not using a SAG contract then the answer to give that
actor is, "We aren't using a SAG contract." What he might be
asking is are you having separate auditions for union and
non-union actors.
Are you using any of the SAG contracts?