Where does &fmt=18 come from?

[brassplyer]

&fmt=18, &fmt=6 - where does the code come from? I know adding it to certain files yields higher quality and stereo if it's available, but why does YouTube recognize this code?

[Soopafresh]

I'm purely guessing, but &fmt=18 is probably pointing to a separate copy of the video, encoded with better settings.