I cels from quite a few different series, and from
what I've seen, it tends to vary from series to
series -- for example, most all my TNN cels have a
secondary number that I've always assumed was a
cut or scene number, since they seem to somewhat
be in order of where they'd be shown (with the
exception of my Eros OP cel, which I'm guessing
may be numbered lower because it's an OP cel).