APP (Atactic Polypropylene) and SBS (Styrene-Butadiene-Styrene) refer to the type of substance that "modifies" the modified bitumen. In the case of an APP modified bitumen membrane, a durable plastic known as atactic polypropylene is added. In the case of SBS modified bitumen membranes, a synthetic rubber known as styrene-butadiene-styrene is added. In simple terms, you can think of APP modified bitumen as plastic asphalt and SBS modified bitumen as elastic asphalt.