The `definexref' destgroup is for the \definexref command
(see Defining generic references). \definexref defines a
destination using the cross-reference label (the first argument) as the
hyperlink label.
The `ref' linkgroup includes \refn and \xrefn (they are
synonyms), \ref, and \refs (see Using generic references).
\refn turns the cross-reference it produces into a link, using
the cross-reference label as the hyperlink label. If an optional
argument is present, it is tied by
\reftie to the reference and become part of the link.
\ref works similarly to \refn. It takes an optional
argument, which is treated the same way as the optional argument to
\refn. In addition, \ref can produce a “class word”.
Both the optional argument and the class word become part of the link,
when present. The cross-reference is tied by
\reftie to the preceding word. The optional argument is
separated from the class word by
\refspace.
Unlike \ref, \refs does not take an optional argument and
does not make the class word part of the link, which is appropriate for
its intended use.