Session is classified into three types,
1. InProc: Which stores the session state on the same web server. This is the only mode that supports Session_OnEnd event.
2. StateServer: which stores session state in a separate process called the ASP.NET state service. This ensures that session state is preserved if the Web application is restarted and also makes session state available to multiple Web servers in a Web farm.
3. SQLServer: Which stores session state in a SQL Server database. Set the sqlConnectionString attribute to a connection string for your SQL Server database.
4. Custom: Custom mode depends on the user choice.
5. Off: There will be no session state.