As nodes can only access their immediate context special object should keep track of path traversal into some context rather than some immediate context. Such objects are called streams . The nodes make up the document's content . Therefore, UNIT is a subtype of NODE . They are terminal units.
Notations are terminal units.
Nodes shouldn't be equalized to units, but have an independent status in the type hierarchy. This is because the node behavior also applies to model units , i.e. objects representing the content model of some element. The UNIT is a way to collect all structural and behavioral features of objects that may occur in a document. All units render services that are based on the fact that they are nodes that are associated with a text portion, have a status, and may predict units in context. The preceding image shows how units make up a document tree.