reftex-default-context-regexps

reftex-default-context-regexps is a variable defined in `reftex-vars.el'.
Its value is

((caption . "\\\\\\(rot\\)?caption\\*?[[{]")
 (item . "\\\\item\\(\\[[^]]*\\]\\)?")
 (eqnarray-like . "\\\\begin{%s}\\|\\\\\\\\")
 (alignat-like . "\\\\begin{%s}{[0-9]*}\\|\\\\\\\\"))

Documentation:
Alist with default regular expressions for finding context.
The form (format regexp (regexp-quote environment)) is used to calculate
the final regular expression - so %s will be replaced with the environment
or macro.

You can customize this variable.