PBF stands for "Purpose Built Fork" and it's a very appreciated, as far as I know. I remember one of my first magazines from back in the day, a 1996 German "Bike Sport News" (one of the better ones in a sea of German hype media), had a test with a full-suspension bike called "Nevil The Cork Gun" and it sported this very fork upfront. Maybe I should take a picture of it...
Mx