One very common requirement in SharePoint, and other portal solutions for that matter, is to have the possibility to target content to a dynamic audience of users and even secure information based on dynamic rules. Traditionally this has been done with Audiences in SharePoint. Audience is a dynamic set of users that is compiled, usually once a day, and at compile time the rules of the Audience is evaluated. A SharePoint Audience is used to target information, but cannot be used to protect content - ie as a security group.