It's usually a 5mm Allen bolt on the outside and slotted lock nut on the inside that needs a special wrench like this to do the job properly. Some chainring bolts have Allen bolts on both sides, sometimes 5mm or 6mm.

With conventional round chainrings the orientation doesn't matter. With non-round eccentric chainrings (Biopace, Absolute Black and others), orientation of the elongated lobes is usually critical to best performance. However opinions differ about optimal position so some folks re-orient the elongated lobes to suit themselves. I've tried that with Biopace but eventually went back to the original factory orientation.