[SOLVED] On bill type change, hidden fields reappear

I don't know if it's an issue, otherwise if it's a question, what is the function called onchange of bill type that make payment fields and other fields appear while I had hidden them?
I tried by searching "idBillType" in all files but I can't find it…

Thanks in advance,

Thanks, I had my own fields table with attributes so I duplicated my hidden fields to the extrahiddenfields table, but I had to add them for every existing types (my fields management system doesn't take into account types).
It gives more possibilities to customize every types, but it is also a limit, if a new type is created we need to add hidden fields all over again!
I tried to put NULL in idType and it doesn't work, is there another way?