When railroads mix the engines in a consist, they will designate the lowest powered engine as the lead engine so that they can pay the crew less money (more for the RR)
The FP7 is 4 feet longer to accomodate a larger water tank for the steam generator. An F7 can very well be equipped with a steam generator, but it's still an F7.
Same goes for FP9 vs. F9.
And there's no such thing as an FP7B or FP9B. Just F7B and F9B units.
Is there room for a steam generator in a standard f7 "A" unit? My understanding was the reason that the Santa Fe used "B" units for steam generators was that there was not room for both a cab and a steam generator in an "A" unit. It would also raise the question, if there is room for a steam generator in a standard f7a, why would anyone make a fp7a 4 feet longer than the standard a unit?