A DSMB or Delayed Surface Marker Buoy works the same as a SMB but it is inflated and deployed from underwater. It is composed of a surface marker buoy secured by a line on a spool or by a WDS. DSMB allow a diver to be located and spotted from the surface while still being at desired depth. It helps in preventing and avoiding accidents with boats and is considered mandatory in many places.
A SMB or Surface Marker Buoy is a safety signaling device in the form of an inflatable tube of fabric brightly colored. SMBs are used at the surface to be easily visible and spotted from far away. Some can also be deployed from underwater and are called DSMB.
DSMB stands for Delayed Surface Marker Buoy and is pretty much the same as a SMB but it is meant to be deployed from underwater. Its warns boats and other people that we are coming up or that we are under. It is also used by technical divers to signal status to the support team.