Sessionstatebehaviour.default vs Sessionstatebehaviour.required

See more:

C#

ASP.NET

C#4.0

MVC

C#5.0

what is difference bbetween Sessionstatebehaviour.default and Sessionstatebehaviour.required, both have read/write access? And why sessionstatebehaviour.readonly allowing me to write value into session as below without any error

[SessionState(SessionStateBehavior.ReadOnly)]
  public class TestController : Controller
  {
      //
      // GET: /Test/

      public ActionResult Index()
      {
          Session["vd"] = "cs";