Let's say i develope a board that has different physical attribute from Arduino board in his shape but that the software would be compatible. Would I be aloud to say on the box that the is compatible with Arduino software?
Yes, you can say that.
The Mini, Pro, Lillypad, Teensy, etc are all considered "Arduino Compatible" in some ways even though they have very different form factors.
Thanx John!
This is the beginning of a new great nightmare for me 
Start with this question and read the next few... I want to design my own board; what should I do?
Well of coarse that the software is free and all so it make it simple to sell. But the main point for me to say Arduino software compatible, is to put the customer in touch with the Arduino community. To me this is the most important part.
I've haven't wrote much on these forum so far, as you can see, but i've learn so much since i've got my first board. And to me making a compatible board and try to commercialise it, it's the next step in my Arduino experience.
Any way's, thank you guy's for the reference link.