PF is "Pool Factor", some thing I've used privately for years, and in my local business, but not here. It's just a ratio between your pool and a pool with 1 million pounds of water.

So . . . if you add 1 pound of something that's 100% concentration, like CYA or chlorine gas, you get a 1 ppm increase of that something (CYA or chlorine). On your pool, since it's about 1/5 the size of a million pound pool, you get a 5 ppm increase.

The calculation works like this:

Say you want to know how much a box (4.75 lbs) of borax will increase your borate level. For complicated reasons, pure borax has an effective borate concentration of 11%. So this is the calculation for YOUR pool: 4.75 x 0.11 x 5 = 2.6. SO, each box of borax will add about 2.5 ppm borates to your pool; to reach a 50 ppm level (effective algistatically) you'd need 20 boxes.

Anyhow, having the PF in signatures allows me to more quickly and accurately calculate doses.