Im guessing that FAA rules say you need to be at 250mph or less below 10k ft, but its so commonly broken that everyone just pretends that everyone is following it.
PM would be the "Pilot Monitoring." On these commercial airlines you will always have a pilot flying (PF) and pilot monitoring (PM). The PM also normally handles communication duties so the PF can focus fully on flying the aircraft.
Me too lol. Running across this via the all feed is a fun thing. It sometimes has drawbacks when some really weird kink community shows up with some unpleasant imagery for my preferences, but that's still kinda cool despite that.