Diagonal checkmate is another type of Corridor checkmate. However, this checkmate is placed only on the diagonal. If the king has free fields (unblocked) on one of the diagonals, then to checkmate him it is necessary to attack not only the king, but also the fields where the king can go on this diagonal.
It would seem that on a diagonal it is possible to attack the king with a bishop (queen), but one of the varieties of Diagonal Checkmate is to set it with a knight (Diagram 2). In this case the knight closes two available fields diagonally.