Subsequently, a unit completely covers the text its sub-units cover. They are the fundamental objects of structuring. SGML general identifiers for elements are mapped onto element types. Elements of the same type within a single document are similar, and should all be implemented by the same class. Specific element types may show specialized behavior, expected from the document substructure they cover.
Documents records a single document element , which is the root of the tree.
Elements not only cover units, they may also reference objects. References to objects are valid only within the framework of the document the referencing units belong to. Reference names are associated with referenced objects by the document . Elements cannot reference elements in other documents. The set of names of external objects differs from the set of element names.