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There will be instances when you need to use subtypes for a class. For example, you may want to use different line symbology for interstate highways and surface streets. But you can also use subtypes to specify a different domain or default value for a field.

Figure 3.11 Subtypes Feature class subtypes can be used to control object behavior. All subtypes share the same attributes and geometry type, but can employ different domains, default values, and rules. Subtypes can improve performance because the speed of most drawing functions is determined by the number of feature classes. Here, we use functional class subtypes to control the default value for RouteDesignator.

In figure 3.11, we use subtypes for a Route table based on the value of the RouteType field. Each subtype will have a different default value for the Route Designator field. This field stores the value representing the entity that can designate a route of that type. For example, the U.S. Route system is managed by the American Association of State Highway and Transportation Officials (AASHTO).¹ When we select a route type for a given row in the Route table, ArcGIS will insert the corresponding route designator value, which saves us a little work. Do this for a whole state, and the ArcGIS subtype function has saved us a lot of work.