The independence of the entity structure is one of the great strengths of SGML, and is essential for hypertext. For example, it relieves an application designer from the burden of having to predict whether chapters will span multiple storage objects or whether a single storage object will contain multiple chapters. For example, the character text within a paragraph, or the raster information representing a photograph, is data. An external entity containing a document that uses a different document representation from the document referencing it is also data. Similarly, a recursive instance of an SGML document or subdocument is data with respect to the current instance, because, as it has its own DTD, it is not parsed in the current SGML parsing context.
Data can also occur outside the content of an element in attribute values or external data entities.
A hyperlink may address its anchors by names that are unique within some name space or by position within a list or tree. A may be expressed in terms of its position and size on the axis, or in terms of its relationship to the address of event B. Spatial alignment is similar, except that the axes are measured in spatial units, rather than temporal. In HyTime, addressing is applied to nodes in groves and the result of any location address is the list of nodes addressed. Syntactically, the list of nodes addressed can be given a unique name by specifying a unique ID for the location address element that addresses the nodes. The nodes exhibiting these properties can then be addressed by name, if they have one, or by other node addressing methods, if supported.