Can't understand the syntax in ocaml code

I have the following code:

module Make_Set (Elt : EQ) : SET with type elt = Elt.t = struct  type elt = Elt.t  type t = elt list  let empty = []  let rec is_element i set =    match set with [] -> false | x :: xs -> Elt.eq x i || is_element i xs  let add i set = if is_element i set then set else i :: setend

I can't understand the SET with type elt = Elt.t, nor I can find it in the official documentation.