Before attempting to understand reusable parts you should be familiar with multi-form applications. This is because you need to understand the concepts of user-defined properties, methods and events.
2.15.1 Why use Reusable Parts?
2.15.2 What Is Using a Reusable Part Like?
2.15.3 When Should Reusable Parts Be Used?
2.15.4 Create a Reusable Part
2.15.5 User-Designed Controls