Fully seam-sealed means every single seam on the jacket is covered with waterproof tape — no exceptions. It's the gold standard for keeping water out, because seams are the weakest points where moisture sneaks in first, no matter how waterproof the fabric itself is.
You'll want fully seam-sealed if you ride in wet climates, chase deep powder, or spend time in the backcountry where getting soaked isn't an option. It's also the right call if you're out all day in stormy conditions and can't duck inside to dry off.
The trade-off is a slightly heavier jacket and a higher price tag. For most resort riders in average conditions, critically taped seams (which cover high-exposure areas like shoulders, hood, and chest) are perfectly fine. But if you've ever felt that mysterious dampness along a seam line on a wet day, going fully seam-sealed eliminates that problem completely.