PPBM is a commercial
aviation acronym which stands for
Positive Passenger Baggage Matching. The idea is that, with
some exceptions, the bad guys don't like to be on the same planes as their bombs.
Maintaining 100% PPBM means that every checked bag loading into an airliner is known to correspond to a specific passenger who is known to be onboard. 100% PPBM is mandated for flights in most of the world, as well as international flights leaving the USA.
Domestic US flights are not currently required to have full PPBM due to the airlines' inability to do it cost effectively. It can also result in significant delays should a passenger decide not to board at the last minute.
The FAA is currently researching different methods of making the PPBM process easier, including RFID bag tagging and other ways of locating and pulling specific bags faster.