What are 'illegal' formations?

[Aku Papua]

I ever played without any single defender at all and used 3 DMF instead and won, it was against a stronger opo..
But i didnt use it at the start of the match (u will lose lots of possesion if u do it at the start) i used it when i was 2 nil down... I pushed all my player forward but without a striker (used amc instead), with normal mentality hard tackle.... Boommmm 2-4... 10 players in the midfield lol