Horizontal carry straps your snowboard across the back of the pack rather than pointing up vertically. The big advantage is a lower center of gravity—your board sits closer to your hips, which feels more stable and balanced when you're hiking or bootpacking steep terrain.
The tradeoff is width. Your board sticks out on both sides, which can catch on tree branches in tight glades or make navigating narrow cat tracks awkward. That's why you'll mostly find horizontal carry on larger touring packs built for open backcountry travel rather than tight resort environments.
Horizontal carry is worth considering if you do longer approaches where stability matters more than maneuverability, or if you find vertical carry feels too top-heavy with your board. Just know it's less common, so your pack options will be more limited.