RBS is a predefined Enterprise Resource custom field and is used in the same
way that other Enterprise custom fields are used. You can use RBS to define
the reporting relationships among users and resources in your organization.
Office Project Server 2007 uses the relationships that are defined in RBS to
simplify the management of access for users and groups. This is an integral
component of resource management and application security.
You can use RBS to define five security rules when you are creating security
categories in Office Project Web Access. RBS is also used in the following
features: Resource Substitution Wizard, Portfolio Analyzer, Build Team from
Enterprise in Office Project Professional 2007 and Build Team in Office
Project Web Access, and two default security categories, My Direct Reports
and My Resources. If you plan to use these features and you plan to use the
Enterprise Resource Pool, then you should consider using RBS.
http://technet.microsoft.com/en-us/library/cc197531.aspx
Hope this helps
Gagan