When answering optimization questions in E6 (D&D3.5 variant), can you assume bonus feats whenever convenient, or only at level 6?

When answering optimization questions in E6 (D&D3.5 variant), can you assume bonus feats whenever convenient, or only at level 6?

I’m very familiar with the underlying 3.5 system, but not very familiar with the conventions of E6. It seems like character-build questions handwave feat requirements away freely, but it’s not clear to me how that works. Can you select arbitrary many bonus feats at any arbitrary level? Or is there some known way to achieve the same effect?

Generally, I would expect this sort of question to be “read the book.” However, E6 does not have a “book” per se (or does it?), and to the extent that it does, that book appears different from how common practice appears.