I have recently discovered the Black Butler fandom, and have noticed that there are exclamation marks between 'Butler' and 'Grell', like this: Butler!Grell. The same is also true for 'Tutor' and 'Sebastian': Tutor!Sebastian. Is there any significance or meaning behind this?
In most fandom the exclamation point is used to indicate a particular style of a character in a series. It's mostly used when characters have episodes or even arcs where they wear a particular outfit like a armor, transformations, or even a disguise.
Most fandom is sustained by avid attention from fans for characters that they like so it helps to distinguish from things like tutor, easter bunny and classic sebastian.
Different fandoms use different notations. But they overall serve the same purpose.