Not really. That's a deliberate thing, because we want to be flexible and things will probably work out differently on a case-by-case basis.
There are some rough guidelines, though, which you can find at:
http://www.d20reviews.com/Natural20/submissions.html
But even those are just guidelines. The idea is that Nat 20 helps you publish your stuff, with you getting involved at whatever level you want. If there is any predominating guideline, Nat 20's main aim is to avoid losing money on a given project - there's no capital behind the company, so if we think a product will make less money than it costs to produce, then we can't afford to do it.