reftex-view-crossref-extra

reftex-view-crossref-extra is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Macros which can be used for the display of cross references.
This is used when `reftex-view-crossref' is called with point in an
argument of a macro. Note that crossref viewing for citations,
references (both ways) and index entries is hard-coded. This variable
is only to configure additional structures for which cross-reference
viewing can be useful. Each entry has the structure

(MACRO-RE SEARCH-RE HIGHLIGHT).

MACRO-RE is matched against the macro. SEARCH-RE is the regexp used
to search for cross references. `%s' in this regexp is replaced with
with the macro argument at point. HIGHLIGHT is an integer indicating
which subgroup of the match should be highlighted.

You can customize this variable.