Application Logoff Page
Use
If an application is closed by an Exit Plug, and no other application is started, the Web Dynpro Framework sends a standard logoff page to the browser. This is entered in the Web Dynpro service node ( /default_host/sap/bc/webdynpro). The Web Dynpro node is the parent node of all Web Dynpro ABAP applications you create.
If you want to now use your own logoff page for your application, you can overwrite this standard page on the subnode of your application.
To do this go to the service node of your application in SICF and enter your own data under 
Error Pages 
Logoff Pages
. You can enter your text on this screen or go to another page.
In stand-alone cases a logoff (deletion of MYSAPSSO2 cookie) can be triggered in the exit plug.
More information:
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Activating HTTP Security Session Management on AS ABAP