Referential. Referential markup refers to entities external to the document and is replaced by those entities during processing. We have already noted the use of referential markup for device-dependent punctuation (e.g., — for an em dash). Another characteristic use if for abbreviations, such as &acm; for “Association for Computing Machinery.” Referential markup might also refer to entities stored in a separate file or even on a diferent computing system.
Most text formatters that support procedural markup offer referential functionality through user-defined “variables” and file imbed or include commands. For the most part, however, referential markup is associated with descriptive markup systems, primarily SGML.