1. A building automation system (BAS) comprising: a database adapted to store data definitions; and a relational directory of data definitions for the BAS stored in the database, the directory including: a site level comprising at least one site definition, the site definition including a site description and a site management description, wherein the site description relates a site with at least one portion of the BAS, and wherein the site management description defines at least one site operation; a system level comprising at least one system definition, wherein the system definition describes an association of a system with a site and an interaction of the system with at least one device comprising a portion of the BAS; a device level comprising at least one device definition, wherein the device definition relates a device with at least one site recognized by the BAS; and an extension level comprising at least one extension definition, wherein each extension definition is associated with a device and defines an association of a device with at least one of a system, a site, or another device, such that the site level, the system level, the device level, and the extension level are organized in a hierarchical relationship in the database.