Seam taping is a process that ensures the effectiveness of a jackets waterproofing. This is achieved by taping, with waterproof tape, behind any stitching on the jacket. This eliminates the ability for the stitching to leak.
Critically Taped jackets are typically taped on the shoulders and around the neck. The means the jacket will not leak in the areas most likely to be getting snowed or rained on. This is typically enough taping to keep you dry on a normal day of moderate activity.
Fully Taped jackets have every stitched seam on the jacket taped. This means that the jacket will not leak anywhere that the fabric has been stitched or pierced by a needle.
Jackets with None mean that no seams have been taped. These jackets are more reasonable in price, but can be prone to leaking at the seams.