Yeah, specifically the PLB3 is an AIS-SART/MOB device (MMSI starts with 972), and it broadcasts “MOB ACTIVE” messages and position reports when activated. Here’s the gory details:I agree this is a bit vague, but I read it to say that the device is sending out an MOB AIS signal.
M.1371 : Technical characteristics for an automatic identification system using time division multiple access in the VHF maritime mobile frequency band
AIS-enabled chart plotters can treat MOB targets differently; one might show a ship icon with the text “MOB”, another might show the official icon for a SART, and another might show the symbol and play an alarm. You can figure out what your chart plotter will do by performing a test, which will broadcast “MOB TEST” messages instead of an active alert.
Unlike the MOB1 the PLB3 doesn’t have DSC, so the above mechanism is how it’s broadcasting a distress signal to AIS-capable nearby boats in addition to the 406 MHz broadcasts to SAR.