Now for the serious answer
PPBM means "positive passenger bag match." If you get this on a baggage tag, your bag is unloaded if you unload or don't load.
International flights are 100% PPBM, domestic is not (yet) 100%, and PPBM happens if you look shady to the computer. Your bag is tagged and is supposed to be loaded last, once you've boarded the plane.
I've only had it happen once, about 2 years ago when I flew one way on UA to LAS, on a full fare F ticket. I noticed that PPBM printed on the bag tag in big black letters. I asked the agent what it meant, and she said she didn't know, just that it happened once in a while.
See:
http://frequentflyer.oag.com/features/f082001-3.asp for more info.
I had "CLR" on my SNA-ORD BP this week too. That one's definately new.
[This message has been edited by skofarrell (edited 10-11-2001).]