I do not know the exact answer for this Q, but Wikipedia has really the same definition that they are applying to go public,
Form S-1 - Wikipedia, the free encyclopedia
Thus, I would imagine they are at the point where they have vested interest to be listed on the big boards versus say as a pink sheet. Furthermore they are obviously trying to raise more funds in the process.
I can't see any other real reason for them to file an S1 unless they really actually wanted to go public.