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)
Some roads - I can't remember which - designated steam generator equipped Fs as F3P, F7P, F9P. Not a manufacturer designation, but useful shorthand.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.
Some roads - I can't remember which - designated steam generator equipped Fs as F3P, F7P, F9P. Not a manufacturer designation, but useful shorthand.