Property sheet extensions are useful for displaying custom properties for messages of a particular message class or for adding your own Microsoft Exchange options. For example:
If your application provides a custom form with custom properties, you can display those properties in one or more property sheet pages. For example, the property sheet of a custom routing form might display the name of the person who initiated the routing process or the routing path of the form.
Your application can supply a custom property sheet that is displayed for certain types of documents. For example, if your extension provides a document archiving system, you might want to provide a property sheet that enables users to select archiving options for documents.