What is the difference between HostingEnvironment class and HttpRuntime class?

As you can see here ASP.NET Application Life Cycle, there are two pictures. On the first there is created HostingEnvironment class in AppDomain, 开发者_StackOverflow中文版on the second is created HttpRuntime class in it. From their definitions i dont understand the difference between them. When is created one and when is created the second during application life cycle? What is the difference between HostingEnvironment class and HttpRuntime class in context of apllication life cycle?

You cannot inherit a HostingEnvironment class and it provides application-management functions and application services to a managed application within its application domain where as HttpRuntime class provides a set of ASP.NET run-time services for the current application.

MSDN