14. Using Windows Forms in VSTO
Office has a user interface that has been designed to make it as easy as possible for an end user to access the functionality provided by each Office application. But the application you are writing that is integrated with Office will have its own very specific user-interface requirements. The application you write will have user-interface needs that are not met by the default Office user interface.
In previous versions of Office, Visual Basic for Applications (VBA) provided the ability to show User Forms to meet your application user-interface requirements. You could also use custom ActiveX controls on the document surface. VSTO adds Windows Forms control support to Office to meet your user-interface ...