Answer

By Country Walking magazine

Boots are rated from B0 to B3 (ask when buying or give the manufacturer a ring if you're not sure of your boots' rating), and crampons from C1 to C3. For boots, the ratings are a reflection of the stiffness of the sole and the support the upper provides; for crampons it's about flexibility and the attachment method - C1 crampons are usually flexible with simple straps, C2 are usually articulated with heel clip, and C3 are completely rigid with heel clip and toe-bail.  

But it's not as simple as matching one number to the other - B1 boots can only be used with C1 crampons, but B2 boots can be used with C1 or C2 crampons, and B3 can be used with C1, C2 or C3 crampons. B0 boots are generally considered too flexible to be used with crampons at all.

And you also need to consider where you'll be going - the more rugged the terrain, the higher the number you'll need. B1 boots with C1 crampons are ok for low level walking, but head higher and you'll need a B2-C2 combination, and for ice-climbing you're looking at B3-C3.