What does the kind field provide in this union?

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; size: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius * shape.radius;
    case "square":
      return shape.size * shape.size;
  }
}