RIS (Rail Interface System) or RAS (Rail Accessory System) is basically similar designation for parts fitted Army Picatinny rail (MIL-STD-1913), or in the civilian sector utilized rail named Weaver.
Both variants are used to attach all the tactical accessories such as sights, front grips, flashlights, AN / PEQ boxes, the stitches on the strap, bipod or even grenade launchers. Options are a handguard for a particular type of weapon or separate universal rails, which can attach almost anywhere.