It's actually the end of that "fmt=18", which is the designator to automatically display in "high quality." I believe "22" is the HD designator.
Embedding may not work properly if, as you said, the video does not possess HD. 18 seems to be "high quality, if not, standard", the HD designator is completely different as you can see when on the youtube site and it displays the larger video frame instead of the standard one.
Some videos have HD, high quality and standard. Some have just HD and standard. Some have just high and standard. YouTube really needs to work on this so it sends the proper stream. (And another thing! It never saves my settings and shows the best quality. I have to set it back every so often. GET ON THE BALL