In the Common Desktop Environment, there are three types of dragable objects: files, buffers, and text selections.
Each application has its own objects that can be dragged and dropped. For example, Calendar uses appointments, Mailer uses mail messages, and File Manager uses folders and files. The folder and file icons in File Manager exist as separate entities in the underlying file system and are, therefore, treated as files when dragged and dropped. However, Calendar appointments and Mailer messages do not exist as separate entities in the file system. When these objects get dragged they are treated as buffers.
This difference can lead to some conflicts for the user. The user sees both of these types of objects as the same -- both can appear as icons and both can be manipulated separately from other similar objects. Yet, due to the implementation, the user may see different results from a drag-and-drop operation based on which type of object is being manipulated.
Text selections fall into a different category because selecting a piece of text is very different to the user than selecting an icon. The user selects a range of text in a document window; the text does not represent the whole document, it only represents a piece of a document. Rarely does a user see the piece of text as a distinct object and the user does not expect a piece of text to behave like an icon when dropped. For this reason, the drag-and-drop model for text mirrors the cut, copy, and paste operations available from the Edit menu.