From Bulbapedia, the community-driven Pokémon encyclopedia.
HMs are multiple use TMs that serve a purpose out of battle, as well as being moves for in-battle use; however, in order to use an HM's purpose out of battle, a Gym Badge is required. HM05 is:
- It is possible that this is because Defog's HeartGold and SoulSilver counterpart, Whirlpool, is a Water-type move.
- HM05 is the only HM and only machine move so far that has changed moves within a generation, from Defog in Diamond, Pearl, and Platinum to Whirlpool in HeartGold and SoulSilver.