I read about the categories in Bob Block's Price Guide. Maybe he came up with the numbers. I don't know. The numbers aren't in common use. For Type I and Type II NLR's, those are common patterns, so we just say NLR and don't give the pattern much thought. I think the Type IV is what we call a Broken Cork, which is more remarkable. But we call it a Broken Cork ... so it doesn't need a number.
The Type III is an unusual pattern, but it never got a name that I know of, so its name became Type III.
(p.s. Wanna see a nice display of broken cork ribbons? ... check this out: http://marbleconnection.com/topic/6786-mostly-pix-peltier/?do=findComment&comment=189627)