Medium volume ski boots have a moderate instep height and internal space through the midfoot area. They're designed to fit the majority of foot shapes—not too tight on top and not too roomy. If you don't have particularly flat feet with thin insteps or especially high arches with thick insteps, medium volume is likely your best bet.
These boots are the sweet spot for all-mountain skiers who want comfort for full days on the slopes without sacrificing control and responsiveness. You get decent heel hold for steering your skis, but enough room to avoid that cramped, numbing feeling that low-volume boots can cause on average feet.
If you're unsure about your instep height, starting with a medium volume boot and working with a bootfitter is a smart move. They can assess whether you need to go up or down in volume based on how your foot fills the boot.