It's a postfix sigil, even if it's part of the identifier name. You're not allowed to put ? in the middle of an identifier. (I use ruby and elixir, so this is second nature to me. It might not be for others).

I'm aware ruby doesn't have fields, I called it fields/methods because it's a method in ruby but people in other languages will read it as a field, which it is, a good percentage of the time.