We also have two styles of 4 mil polyester sleeves. They are both made of the same archival polyester material, but they have two different flap styles: • The Open Flap style (ROF) has to be creased to form the flap and then folded over. • The Beaded Flap style (BF) has a third piece of polyester that is bead welded to the extended back of the sleeve, creating the flap.