#### Extension

The Extension drop-down list becomes available when the item chosen in ID drop-down list requires the addition of an extension in order to form a useable expression element. The Extension drop-down list displays available extensions based on the item selected in the ID drop-down list.

Example: Suppose you select a feature from the ID drop-down list. The list of possible extensions that can be used to reference data of that feature (such as "X", "Y", "Z", "Diam", "Length", etc.) are then listed in the Extension drop-down list.