Separating zippers are zippers that open at the bottom allowing both sides of the zipper to come apart as in a jacket zipper. The type of separating zipper depends on the garment you will be using the zipper.
Length and teeth weight are the primary considerations. A metal zipper will weigh more than a polyester or plastic tooth zipper. For example, a light weight fleece jacket would sag with the weight of a metal zipper. A heavy weight wool jacket would welcome the strength and rigidity of a metal zipper and would not sag from the weight of a metal zipper.
Separating zippers are also available in a reversible zipper.