It's to denote the difference between an FP1/2/3 service schedule and a P1/2/3 schedule.
<18,000 miles per annum follows the P schedule.
>18,000 miles per annum follows the FP schedule.
I don't know off hand what the difference is between the two, but it's in the service book - I presume the P schedule is pages 18/19, the FP schedule is pages 20/21 - but I'm far too lazy to go and look.
As far as I can remember they're both 9k intervals.