The MFC library supports two distinct application types: Single Document Interface (SDI) and Multiple Document Interface (MDI). An SDI application has, from the user's point of view, only one window. If the application depends on disk-file "documents," only one document can be loaded at a time. The original Windows Notepad is an example of an SDI application. An MDI application has multiple child windows, each of which corresponds to an individual document. Microsoft Word is a good example of an MDI application.
When you run AppWizard to create a new project, MDI is the default application type. For the early examples in this book, you'll be generating SDI applications because fewer classes and features are involved. Be sure you select the Single Document option (on the first AppWizard screen) for these examples. Starting with Chapter 18, you'll be generating MDI applications. The MFC library application framework architecture ensures that most SDI examples can be upgraded easily to MDI applications.