向 C# ASP.NET Core 管道添加中间件时，“Map”扩展有什么用？

private static void HandleMapTest(IApplicationBuilder app){
   app.Run(async context =>{
      await context.Response.WriteAsync("Map Test Successful");
   });
}
public void ConfigureMapping(IApplicationBuilder app){
   app.Map("/maptest", HandleMapTest);
}

private static void HandleBranch(IApplicationBuilder app){
   app.Run(async context =>{
      await context.Response.WriteAsync("Branch used.");
   });
}
public void ConfigureMapWhen(IApplicationBuilder app){
   app.MapWhen(context => {
      return context.Request.Query.ContainsKey("branch");
   }, HandleBranch);
      app.Run(async context =>{
         await context.Response.WriteAsync("Hello from " + _environment);
   });
}

app.Map("/level1", level1App => {
   level1App.Map("/level2a", level2AApp => {
      // "/level1/level2a"
      //...
   });
   level1App.Map("/level2b", level2BApp => {
      // "/level1/level2b"
      //...
   });
});