Daft question, I'm sure..!

I've a Q7 (4m) which has Audi OEM cross bars to affix a roof box. Having never owned one before I'm confused as to what to look out for regards roof box fixings. Are specific boxes made for specific models or, is there an industry standard? A scan of the internet hasn't really clarified things for me.

Also, I've a friend who has offered me the loan of his BMW box used on an X3 - would that work?

cheers in advance for any guidance offered