Maybe the question have been already asked before.
I didn't find any topic performing a search on 'using' keyword.

Here is the little question:
If I have this query
During the 'c' table join the 'y' field will be used for joinning with
- a
- b
or both ?

that's a good question and one of the reasons why i recommend that people should not use USING, just code the conditions explicitly

That's what I thought.
So USING and NATURAL are to be banned.
I'll never use them, even when I can.
Because I think we must code always the same way.
Do you know a good site where I can find a norm or something like that, which says the things to do and not to do ?

We all need norms

i know what you mean, that is often called "best practices"

unfortunately this can only be mastered with experience, as all situations are different and what you do in one case is not necessarily what you'd do in another

I agree.
But I think we can easily say for example that we have to ban certain fonctions.
In my school, I have been imposed norms for C, and it have been proven usefull in term of optimisation, interoperability, maintenance and speed of developpement.