Although the move it contains varies between generations, it has always been the only HM to teach a move that deals no damage.
It is also the only multiple-generation HM that is not truly required to move on.
HM05 is the only multiple-generation HM that is never received in a city.
In Diamond and Pearl, HM05 has the item sprite of a Water-type HM, even though Defog is a Flying-type move.
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