HMs are TMs that serve a purpose out of battle, as well as being moves for in-battle use; however, with the exception of Generation V, in order to use an HM's purpose out of battle, a Gym Badge is required. In Generation VII, HMs are not used and HM05 does not exist. In other generations, HM05 is:
HM05 appears as a Water-type HM in Diamond, Pearl, and Platinum.
In Diamond, Pearl, and Platinum, HM05 has the item sprite of a Water-type HM, even though Defog is a Flying-type move. It is possible that a Water-type HM was considered for the position before Defog was finalized. Whirlpool, a Water-type move itself, takes the spot in HeartGold and SoulSilver, though this is likely unrelated, as HeartGold and SoulSilver came out more than three years after Diamond and Pearl were released.
HM05 is the only HM that has changed moves within a generation, from Defog in Diamond, Pearl, and Platinum to Whirlpool in HeartGold and SoulSilver.
HM05 has contained the most different moves of any HM, teaching four different moves throughout the series.
Interestingly, two of the moves it has contained are the two HM moves which first appeared as HMs in Generation II.
HM05 is the first and so far the only HM that has been handed out as a prize for defeating a Gym Leader.
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