Almost all interfaces today, with the exception of TheBrain visual user interface, are limited to organizing information into hierarchies, where a piece of information can only be categorized into one place. For simple applications this is fine, but for users engaging in more complex business processes, it is simply inadequate. A document will have a variety of different issues or people associated with it – with hierarchies one cannot show all these relationships without multiple copies of the information.
Shelley Hayduk also identifies the issue that most information management software uses a file cabinet metaphor (i.e. hierarchy). This has the limitation that a piece of information can only be categorized in one place. For more complex things, this is inadequate.