14. A BMS system according to claim 6 comprising: data indicative of a plurality of facility model items which respectively describe physical locations within a building environment in terms of a category of location, and relationships with other locations, wherein a category of facility model items describe locations uniquely serviced by respective pieces of HVAC equipment; and data indicative of plurality of equipment model items, each equipment model item describing a specific piece of HVAC equipment in terms of a category of equipment selected from a set of available predefined categories of equipment, one or more data points associated with that piece of equipment, the roles of those data points, and relationships with one or more further pieces of equipment, wherein for at least one category of equipment the equipment model items include data indicative of a location uniquely serviced by that piece of equipment; such that the facility model items and equipment model items are linked by the locations uniquely serviced by respective pieces of HVAC equipment.
14. A BMS system according to claim 6 comprising: data indicative of a plurality of facility model items which respectively describe physical locations within a building environment in terms of a category of location, and relationships with other locations, wherein a category of facility model items describe locations uniquely serviced by respective pieces of HVAC equipment; and data indicative of plurality of equipment model items, each equipment model item describing a specific piece of HVAC equipment in terms of a category of equipment selected from a set of available predefined categories of equipment, one or more data points associated with that piece of equipment, the roles of those data points, and relationships with one or more further pieces of equipment, wherein for at least one category of equipment the equipment model items include data indicative of a location uniquely serviced by that piece of equipment; such that the facility model items and equipment model items are linked by the locations uniquely serviced by respective pieces of HVAC equipment. 16. A computer system according to claim 14 wherein: a facility model is configured to provide a graphical representation of the facility model items via a tree structure, and an equipment model is configured to provide a graphical representation of the equipment model items via a tree structure, such that the facility model and equipment model being linked by the locations uniquely serviced by respective pieces of HVAC equipment to provide a graphical representation on the display of the facility model items and equipment model items via a common tree structure.