DSMB is one of the first gear to own in diving. It's primary purpose is to provide surface signaling. This helps reduce risk of drifting further away from boat or shore unnoticed, provide signal for watercraft vehicle traffic to avoid the area.
 
This semi closed design helps keep air on surface and could provide extra buoyancy both surface and underwater. Inflation can be made with second stage, oral inflation, or inflator hose which meets every diver's demand.