Looking at flights today I noticed that some of the options show availabilty of P (eg, P4, P8) instead of F. Price out SFO-JFK and you'll see what I mean. The 3 class planes seem to show:

P4 C9 Y9 etc

whereas the 2 class planes show:

F9 Y9 B9 etc...

Just a glitch? random? or is this a new code?