You can create a new part directly from an existing part. The new part, called a derived part, has the original part as its first feature, and it is linked to that part by means of an external reference. This means that any changes you make to the original part are reflected in the derived part.
When a part has an external reference, its name in the FeatureManager design tree is followed by an arrow ->. To see the name, location, and status of the externally referenced document, right-click the derived part, and select List External Refs.
There are three types of derived parts: