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You can add new edge and junction feature classes at any time in a geometric network's life. These new feature classes are empty—you cannot add populated feature classes to an existing geometric network. Adding a new feature class to a geometric network is similar to the task of creating a new feature class.
When creating a new network feature class, you must specify whether you want to add a simple junction, simple edge, or complex feature class and specify the geometric network in which that feature class will participate. The new feature class must be created in the same feature dataset as the geometric network.
All network feature classes have the same required fields as simple feature classes—OBJECTID and Shape. In addition to this, network features have a required field, Enabled. The Enabled field records whether a feature in the feature class is enabled or disabled in the logical network. This field has an attribute domain automatically associated with it by the geodatabase.
If you create a new junction feature class, you can specify that its features be able to act as sources or sinks. To record whether a junction feature is a source or a sink, the required AncillaryRole field is created for the feature class. Like the Enabled field, the AncillaryRole field has an attribute domain automatically associated with it.
Learn more about sources and sinks