Hypertext modelling specifies composition and navigation of the site.
Composition describes which pages compose the hypertext, and which content units make up a page. The pages of the Web site are the containers of information actually delivered to the reader.
Units are atomic content elements used to publish the information described in the data model. Seven types of units are predefined in WebML to compose pages: data, multi-data, index (and its variants multichoice and hierarchical), entry, scroller. Each unit is associated to one underlying entity, from which the content of the unit is computed. The specification of the underlying entity dictates the object type from which the content of a unit is derived (e.g., albums, artists,...). When appropriate, units may be optionally associated to a selector, i.e., the specification of a set of restrctions that determine the actual instances of the underlying entity to be used as the content of the unit at runtime.
Navigation of the site is specified thru links. Links can be defined between the units inside a single page, between units placed in different pages, and between pages. The information carried along a link is called navigation context, or simply context. Links that carry context information are called contextual links, whereas links that have no associated context information are called non-contextual links. Context information is typically necessary to ensure the computability of units.
The figure below reports and example of WebML hypertext specification.
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