What is a Global.asa file

Hello everyone!
The Global.asa file is an optional file in which you can define event scripts and use session and application objects. The contents of the Global.asa file cannot be displayed to the user, but the information it stores can be applied to the entire application. This file must be named Global.asa and stored under the directory of the application's startup point, an application can have only one global.asa file.
The Global.asa file can only contain the following:
1,application event.
2,session event.
3,<object> 's statement.
4, the class library declaration.