You might be confused by all this talk of fretboard radiuses, but you needn't be. Radius refers to the curvature of this vital part of your guitar. Some guitars (like certain vintage Fenders) feature rounded boards, whereas others are near flat. While each has its own benefits, suffice it to say that different fretboard designs represent a compromise in capo design.
Fix a rounded capo on a flat fretboard (or vice versa) and you’ll end up with unwanted string buzz (the noise made where strings aren’t fully clamped) and poor tuning (where strings are pushed sharp from over-tightening the capo). In a perfect world, every capo would apply even force across all six strings (and at every fret position), in order to prevent these problems.
Generally, capos can be divided into radiused capos for rounder fretboards; flatter designs, typically for shreddy SuperStrats and classical guitars; and all-in-ones – capos that either accommodate different radiuses (such as the G7th) or incorporate both a flat surface and a rounded one into their design.