In general, the nodes and the links between them can have multiple attributes. The entire structure of the hierarchy and its encompassing relations is also usually relevant. Tasks can be applied to a single node, a link, a collection of nodes, or even to the entire structure.
Hierarchical data is very diverse and is encountered in many forms.
Hierarchies naturally arise in taxonomies, the structures of organizations, disk space management, genealogies, and the Dewey Decimal system. Representing trees in this manner severly limits both the depth and breadth of the tree that one can view at a single time. Furthermore, navigating and finding specific nodes in such a structure can be confusing, disorienting and downright frustrating. More recent visualization techniques attempt to show many more nodes, if not the entire tree itself, as well as providing mechanisms for navigation and searching which allows the user to keep the context of the entire tree in mind as well as reducing the disorientation. Hierarchies can be seen as a special case of networks, except the definition of hierarchies eliminates the possibilities of dual paths and cycles.