URL filtering allows you to create access rules that allow or block access to Web sites based on their categorization in the URL filtering database. When a request to access a Web site is received, Forefront TMG queries the remotely hosted Microsoft Reputation Service to determine the categorization of the Web site. If the Web site has been categorized as a blocked URL category or category set, Forefront TMG blocks the request.
If a user requests access to a Web site and discovers that access to the Web site is blocked, he receives a denial notification that includes the denied request category. In some cases, the user may contact the administrator to dispute the categorization of the Web site. In such a case, you must check that the URL was categorized properly (see Looking up a URL category). If the Web site was not categorized correctly, then you must create a custom setting for this URL (see Overriding URL categorization).