Controls how large the neck hole is. Keep in mind that most garments have neck holes that are substantially larger than the wearer’s neck, and that the neckband will further constrict the neck. 0% ease will make a very small neck hole that hugs the neck snugly. +50% will still make what’s generally considered a tight neckband appropriate for swimwear. For garments with a loose fit, or that do not use a zipper, even larger values are recommended.
Be careful setting a negative ease for the neck, even on compression garments, as it can get very uncomfortable. Perhaps it could be useful for a garment trying to seal off the body, such as a wetsuit?