A liner. They’re all virgin vinyl and winterized. Non-virgin vinyl (reprocessed) would have dirt and pieces of different vinyl mixed in, which would leave little holes in the vinyl. Even the worst liners made are still pretty virgin – maybe with some scraps of brand new liner melted down and mixed in.
There are, however, some very important differences in liners. Generic liners are more likely to wrinkle because they are made to a wider tolerance (to fit pools with differing shapes and final sizes). If a defective liner causes a wall to split, it’s a lot easier to collect when they’re both from the same manufacturer.
Companies that only make generic liners (without pools) are under tremendous pressure to make them as cheaply as possible. Liners are not made just out of vinyl. They have colorants to make them blue, U.V. inhibitors to keep the colors from fading, and plasticizers to keep the vinyl from becoming brittle with age and cold. Mildewcides and algaecides keep microscopic plants from forming roots into the vinyl. Elasticizers allow the liner to stretch smoothly for a wrinkle-free fit. Some seaming techniques are much stronger than others.
Gauge has no real meaning in measuring a liner’s quality. To a large extent, neither does warranty unless you read the warranty carefully. If it’s prorated properly there’s not much warranty left after three years – whether it’s called ten years, thirty years, or triple lifetime! Does it cover just the seams or the entire liner?
Years ago, 20 gauge meant at least 20 thousandths of an inch thick, or 20 mil. Today, that same 20 gauge liner probably weighs less than it used to. Ask to see and feel a sample of the liner that comes with your pool. Ask for the actual, true mil thickness of your liner, in writing. It will probably look like a fraction, with the top digit representing the sides and the bottom digit representing the bottom.
Liners are often made with different material on the bottom than on the sides. The bottom generally is not required to stretch very much during the installation, while the sides are often stretched during filling and wrinkle removal. The sides often have extra ultra violet protection, because the top of it is exposed to constant sunlight. The sides are usually thicker, too.