Surface marker buoys (SMB): Good for signaling boat drivers of your location while performing the safety stop or ascending. SMB makes boat drivers see you from far at the surface. The Yellow SMB it is used more commonly in tech-diving and means emergency underwater/ need assistance (for those divers that have to spend hours on decompression stops). - See more at: http://www.scubadivingfanclub.com/Diving_Signals.html#sthash.4FNECam2.dpuf