IOS空协议，然后拓展协议，拓展协议的self值得是什么？

Question

protocol TableViewCellFromNib {}
extension TableViewCellFromNib {
    static var identifier: String { return "\(self)ID" }
    static var nib: UINib? { return UINib(nibName: "\(self)", bundle: nil) }
    static func hw_getNibPath() -> String? {
        return Bundle.main.path(forResource: "\(self)", ofType: "nib")
    }
}