PF is the ratio of your pool to a million pound (PF:1) pool. 1 pound of something with a 100% concentration (like good stabilizer) will result in a 1 ppm increase in levels in a million pound pool, but in a 12 ppm increase in a PF:12 pool.

It's something I've used for years, but only started using here this summer. (I need to write a "How to Use Your Pool Factor" page )