No the prefix is the first four letters of the code. Vudu has a limit of 5 per prefix on most items, 4 on some like the TMNT pizza promotion, and 2 on certain items. People are asking to make sure that they haven't reached their max remediation allowed by vudu. Disclosing the prefix has no hinderance on the validity of the code at all but to let buyers know if they can actually bid or pass on your auction.
I'm not sure what you mean.... the first 4 letters determine how many of that particular group of codes you can use. if I've already used 5 codes with that particular prefix, I wont be able to use them since vudu puts a limit on that. if you look at other peoples auctions for vudu credits the first 4 letters are listed so people know whether they should bid or not