All assets not from the vanilla games must be credited to their creators, obviously.
F2U means “free to use” – you are allowed to use it, and you are allowed to recolor it, but you are not allowed to edit it beyond recoloring, or use it in splicing or whatever, without direct permission. Everything in the Graphics Repo is at minimum F2U.
F2E means “free to edit” – you are allowed to use it, edit it, splice it, etc. in any way so long as you credit its creators (you must still credit the original creator after editing!). It is strictly broader than F2U, unless specified otherwise by the artist. Specifically animations in the Repo are F2E by default, other Repo assets can only be assumed F2U unless stated as F2E.
You can read more in detail here: Regarding graphics assets - #4 by HyperGammaSpaces