What does being typeless do in game?

Question

Was reading about the move Burn Up and from what I've seen, elsewhere and on this site, it leaves pure Fire-type Pokemon typeless upon execution.
What does being typeless do in-game? Is it treated like a normal type with it's weakness, immunity and STAB? Is it neutral to all attacks and not have STAB to any of it's moves?

Answer 1

It is neutral to all attacks and loses STAB. However, it cannot use burn up again.
Source

Comments

Thank you.

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your welcome. If you'd like to know, Roost removes flying type from flying type Pokemon only the turn it is used, which is why nearly all flying types were dual type. Then it changed to making the Pokemon normal type if it lost all of it's typings. Even then, game freak left all flying-normal Pokemon the same and still haven't made another pure flying type other than Tornadus.