Mountaineering boots fit into 3 categories for crampons:
B1 boots that are generally 3-4 season hiking boots will take a strap-on crampon (C1)
B2 boots are rigid mountaineering boots with a heel welt that will accept a hybrid crampon (C2) or a strap-on crampon (C1)
B3 boots are rigid winter mountaineering boots and ice climbing boots that have both a heel and toe welt. These boots accept an automatic crampon (C3), a hybrid crampon (C2), and a strap-on crampon (C1).