coref — A cross-reference to a co
coref ::= empty
coref element plays a role for callouts
that is analogous to the role of
footnoteref for footnotes.
co and one or more
coref elements when you want to indicate that the
same callout should appear in several places.
coref is not a cross-reference to a callout
xref for that); rather, it is an indication
that the callout appears semantically in more than one place.
Specifies an identifying string for presentation purposes