This tag is to be used when a zipper in the image (usually on clothing, but applicable elsewhere) has been fully opened, a.k.a. unzipped.
If the zipper is still in the process of being opened, use unzipping instead.
The 下ろしチャック (lowered zipper) tag on pixiv is often used to refer to a type of unzipped fashion where the chest is left exposed with the nipples covered, oftentimes with the navel also shown.
Nero Claudius (Bride) is a character often demonstrative of this feature, such as in post #2501105.