Defining the Portal Gateway Mechanism 
The portal gateway mechanism serves as an entrance to the portal. It allows you to modify the behavior of the portal by:
Customizing URLs to portal applications
Using aliases for custom URLs
You can assign names (aliases) to custom URLs in the portal to enable easy access to portal content via different URLs
For example, having defined more than one entry URL, you can configure the gateway to direct a URL to a high or low speed connection.
Defining suitable portal pages for users based on their authorization level and authentication schemes
For more information, refer to the section Defining a Portal Gateway Mechanism in the How-To Configure the J2EE Engine Deployment Descriptor guide located on SAP Developer Network at .