A top-loading ski backpack uses a single opening at the top of the pack, which is the most traditional and straightforward design you'll find. It's popular for a reason: with fewer zippers and seams, there are fewer points where the pack can fail or let moisture in. That makes top-loading packs naturally more weather-resistant and durable—big advantages when you're dealing with snow and wet conditions all day.
The trade-off is access. If you need something buried at the bottom of the pack, you'll have to pull out everything above it first. This can be frustrating if you carry lots of layers, food, or gear you want to grab quickly throughout the day.
Top-loading shines for backcountry touring where simplicity and weather protection matter most, and where you tend to pack in a deliberate order—safety gear in dedicated pockets, items you'll need first near the top. If you prefer rummaging through your pack or need frequent access to multiple items, consider a pack that combines top-loading with a side access zipper for the best of both worlds.