Is a move considered a "signature move" if more than one Pokemon learn it?

Question

I don't mean (or at least mostly don't mean) totally unrelated Pokemon. It's more of these, like "is it a signature move if the whole line learns it?"

Question one: Is it a signature move if the whole line learns it, as opposed to the last stage learning it upon evolution? For example Bonemerang is learned by the whole Cubone line, as opposed to just Marowak. Is Bonemerang a signature move?

Question two: Is it a signature move if both normal and regional form learn it, even though they're technically the same Pokemon? For example, regular Tauros and Paldean Tauros: they both learn Raging Bull. Is Raging Bull a signature move?

Question three: What if multiple Pokemon learn it, but it's just kind of one Pokemon's "thing"? For example, Water Spout is almost always associated with Kyogre, but (according to Bulbapedia, not this site) both Jellicent and Wailord learn it, as well as their first evolutions. Is Water Spout a signature move?

Comments

There's no official list of signature moves. You can define signature moves however you want.

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there is a list on bulbapedia of all the pokemon with moves that are only theirs though... and cubone and marowak are on it. I mostly just wanted to know how people thought about it.

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There's some controversy over the difference between signature moves (moves that are designed around the concept of a particular Pokemon, usually released with that Pokemon, but it is not unique to that Pokemon) and Signature Moves (mostly associated with a legendary or mythical Pokemon, and "belongs" to that Pokemon, usually only learned by that Pokemon (and Smeargle, who cannot use it)).  The differences have degraded over time, as a few Signature Moves have been given to different but related Pokemon (ex V Create).  Meanwhile, some signature moves have really never been given to any further Pokemon than the one it was concepted for (ex Water Shuriken).

Realistically, Game Freak kind of needs to make a decision about these poorly defined move classifications, and pick 1 strategy.

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Why should they? Game Freak just gives moves to Pokemon. They never defined these words.

Answer 1

what I think of it is that when a brand new Pokemon is introduced with a special move to go with I call that a signature move, at least until other Pokemon learn it,
for example my good spiky boy Chesnaught.

He learns Spiky Shield upon Evolution and is the only Pokemon in Gen 6 that learns this move

however in Generation 7

Togedamaru learns Spiky Shield at lvl 49 making it no longer Chesnaught's Signature move.
To answer your question about Marowak and Cubone I think it is His line's special move since it is only confined to his evolution family, alongside Tauros who even though he has a Paldean form is still technically the same Pokemon so I would also call Raging Bull Tauros's signature move since only his species learns it, I'm sure this will change in the next generation of Pokemon games as some Pokemon will likely have their moveset updated with this move who knows maybe Miltank will learn it in Gen 10