It's a factor for calculating your chemical doses. For example, stabilizer is usually 100% concentration, so 1 lb of stabilizer will add 24 ppm of CYA to your pool. And, a gallon of 6% bleach has about 1/2 pound of chlorine gas (100%!) in it, so a gallon of bleach will add about 12 ppm of chlorine (24 x 0.5) to your pool.