The SGML standard defines three flavors of link, which differ according how explicit they are about the nature of the result document type. The intent of simple link is to enable the setting of global style information for the whole document. For example, a simple link process might name the style sheet to be used with a document or set the base font and font size. Defines a mapping from elements in the input document to elements in a result document. Attributes can be associated with both the input elements and the result elements.
Implicit link serves to associate link attributes with elements in the input document.
Each link rule associates an element in context with any link attributes and, for explicit link, the result element type and attributes. A document may have one or more link processes associated with it, but only one can be active at a time. It is up to the processing application to determine which link process is active when parsing starts. Link is important because it defines a standardized way to do hierarchical association of style to elements using SGML-defined mechanisms. For this reason, it serves as a bridge between the SGML parsing of a document and the processing done by specific processing systems.