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:
- 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.