Single document: The standard mode of notepad programs, such as menu bar and toolbar, can only operate one document, that is, you cannot open two files in the same application at the same time.
Multi-document: Standard mode of WORD or browser program, which can display different information in multiple windows, perform different tasks, have multiple view environments, and can operate multiple files at the same time. Multiple files share the same menu bar, toolbar, etc.
Dialog: easy to use controls, WYSIWYG programming, no menu bar, toolbar, etc.
