Actually for a B16 versus a B18c, the head designs are different. For example, B16 cams will fit in the B18c5 head. For some reason I seem to remember hearing someone use B18c cams in a B16 though...

Cams are catalogged based on their total lift, average duration, timing of each and material type. Obviously some cams will be better suited to larger engines than others, but that is not the sole factor in how they are broken up. It's more like a suggested use, they know the cams will work with a certain engine, if you want to experiment, you do so at your own risk.