A DGN file contains both graphics and business data. You see the graphics displayed in the drawing and the business data appears in the Explorer dialog. Business data is the real world information associated with the items represented by the graphics in the drawing.

If you consider a DGN model representing a facility, the facility is made up of pumps, vessels, valves, pipelines, and so on. These items can be represented graphically but they also have associated business information that defines the nongraphic properties. Any of these graphics will contain properties which define the graphic such as size, manufacturer, material, or design pressure.

Business data includes:

• Item Types — Item types define the business properties for a specific type of item.
• Items — Items are the individual representations of an Item type. For example, a facility might have a Centrifugal Pump Item Type defined but have multiple pumps represented in the design. Each pump would have the same properties but different values of these properties.
• Relationship Types — Just as items are categorized by their types, relationships are also categorized by types. The relationship types generally convey some information about the nature of the relationship between the items. For example, a Pump item might have an is annotated by relationship with an annotation item. Similarly, it may also have an Equipment Has Nozzle relationship with a Nozzle item.
• Libraries — Libraries hold a collection of Item Types and Relationship Types, usually related to a specific domain. You might have one library with Building Item Types and another Library with Plant Item Types.

When you click an item in the Items tab, the Details dialog or Properties dialog, if open, displays the properties associated with that item.