External database links are most appropriately used when data is dynamic in nature. For example, a sewer pipe may have several instances of maintenance. This is an ideal situation in which to use an external database link. The object has a single key, which is related to a record in a database. The database application can then be used to relate this record to other tables that contain additional information.
Some of the common benefits of using external tables:
1. External data can be accessed by individuals without their having access to the drawing files.
2. External data is better for data that changes frequently, such as maintenance information.
3. Drawing file size is not increased if the data is stored externally.
When you drag and drop a database file into the AutoCAD Map 3D Explorer task pane to attach it to the drawing, a Universal Data Link (UDL) is automatically created. By default, it is stored in the folder called Data Links in the Application Data folder in the users Document and Settings folder.