B0 Mountaineering Boots
Flexible, comfortable and reasonably light a B0 boot is ideally used for general summer hillwalking and well-pathed hiking. They are not that suitable for winter walking as even the most flexible crampons do not flex enough to stay attached to the sole of the boot. The straps on crampons would need tightening so much there is a high chance over a few hours the boots would become damaged.
B1 Mountaineering Boots
A good all-round walking boot will have a B1 rating. You can expect a slightly stiffer midsole to combine with flexible C1 walking crampons, uppers are typically more insulated and modern boots have a rubber rand around the toe to protect the forefoot whilst scrambling. B1 boots are lighter than B2 and are ideal for crossing simple glaciers to get to Alpine rock routes or all-season hill walking in the British mountains.
B2 Mountaineering Boots
The most common rated boot for summer Alpine mountaineering, Scottish intro winter mountaineering and regular glacier travel. B2 boots provide more substantial insulation and a stiffer sole for kicking steps, they combine with a C2 and C1 crampons for more regular winter/glacier use by featuring a horizontal welt in the heel. Modern B2 boots are light, have fabric uppers and scramble efficiently. they are a good ‘jack of all trades’ in the UK mountains and for summer Alpine mountaineering but do not cope as well as a fully-featured B3 boot on steep ice or in very cold temperatures.
B3 Mountaineering Boots
In my opinion, this is where the widest variety of boot is categorised as B3 boots can be light and minimalistic whilst still being stiff enough for the most technical ice fall climbing but are also the most heavily insulated boots built for Everest and the Polar regions. It is, therefore, the most difficult category to choose a boot from as many appear very similar and have identical features but neither one nor the other will be suitable for your chosen discipline. Scottish winter climbers will seek a very waterproof boot with a stiff and solid outsole to cope with the variety of ground and turbulent conditions whilst climbers departing on Himalayan Expeditions may require the warmest double boot that has a softer sole as they will more regularly be travelling on snow and ice. Most modern B3 mountaineering boots have welts in both the toe and the heel to accommodate clip in technical C3 crampons.