One of the benefits of a derived part is that you can use the derived part to provide base geometry quickly. Then add new features to that base geometry. In effect, you use the source file geometry as a foundational starting point for your design work. At the same time, you preserve the design state of the source file. There is a one-way associative link between the source file and the derived part. Any changes made to the source file can be applied to the derived part. However, changes made to the derived part do not modify the source file.