You can create extensions that use either modal or modeless windows. When a modal window is displayed, users can only interact with the extension and not with Microsoft Exchange. For example, an extension might display a modal Folder Backup dialog box that enables a user to choose a specific folder to back up.
In contrast, some extensions can run modelessly, displaying windows that allow the user to switch their input back and forth between Microsoft Exchange windows and the extension's windows. For example, a Stock Quote extension could display a modeless window that displays dynamically changing stock data while the user interacts with Microsoft Exchange windows.
For simplicity, an extension that displays only modal windows is referred to as a modal extension while an extension that displays at least one modeless window is referred to as a modeless extension.
Some extensions can involve a combination of modeless and modal windows; that is, modeless extensions might display modal windows at certain times. For example, the Stock Quote extension mentioned previously might display a modal Options dialog box.
When writing modeless extensions, it is important to coordinate the actions of your extension's windows with the windows of Microsoft Exchange. For example, if Microsoft Exchange displays a modal window, modeless extensions should disable their windows. Interfaces that enable this cooperation are implemented by Microsoft Exchange and extensions.