%version

version (version) defines the name to be used for the root element of a concurrent markup stream for marking pages and lines of some reference edition.
Note

Some name for the edition should be supplied by defining this parameter entity within the appropriate DTD subset. If none is defined, the parameter entity (and thus the document type) default to ‘ref’.

Declaration <!ENTITY % version 'ref' >
See further 31.6 Concurrent Markup for Pages and Lines