Joe- Child numbers are grouped vaguely by type, but only vaguely. Robin Hood ballads, for example, are listed more or less consecutively. Each number, though, was meant by FJC to be a unique ballad. Trouble seems to be that when he did this only God and Francis James Child fully comprehended his system. Now, Child is dead.
As far as Laws is concerned, He used a letter/number combination in which the letter was supposed to be a descriptor of the type of ballad, and the numbers were simply consecutive examples. Trouble is, Laws never explained his basis for separation, and used examples that aren't readily available.
Susan spent an unGodly amount of effort trying to p[rovide examples of all the Laws numbers in DigiTrad, but I don't think that even God knew why there are five or so "Plains of Waterloo" listed by Laws, or what distinguished one from another.
Still trying to find out where Little Susie came from. She appears in one version Sharp collected in (I think) Kuntucky, and in Linscott's Folk Songs of Old New England.