It is a good idea to keep track of what movie it came out of, because people can only enter each movie once per account, if that makes sense. So it is safer to bid if it is for a movie you know you have not already entered yourself. :)
Right, it is a rewards code, but they are found in movie boxes, and a person can only enter a code from inside a box of any particular movie once. So if your code is out of Cars, for example, but I have already bought Cars and entered the code I found in my package, then your code will not work in my account. Make sense? That's why people ask where you got it.