The FAQ You Requested...
What is URL Masking and when would I use it?
URL Masking works in conjunction with Web Forwarding to help hide the fact that a particular Website doesn't exist.
For example, if you own two similar domain names and don't want to incur the expense of creating duplicate websites, you can build the site using one of the domain names and use the web-forwarding facility to ensure that users will find the site even if they type in the alternative domain name. The "masking" facility will hide the fact that the website being viewed isn't exactly the same name as the one requested.