RefTeX needs to be aware of the environments which can be referenced with a label (i.e., which carry their own counters). By default, RefTeX recognizes all labeled environments and macros discussed in The LaTeX Companion by Goossens, Mittelbach & Samarin, Addison-Wesley 1994.. These are:
figure, figure*, table, table*, equation,
eqnarray, enumerate, the \footnote macro (this is
the LaTeX core stuff)
align, gather, multline, flalign,
alignat, xalignat, xxalignat, subequations
(from AMS-LaTeX’s amsmath.sty package)
\endnote macro (from endnotes.sty)
Beqnarray (fancybox.sty)
floatingfig (floatfig.sty)
longtable (longtable.sty)
figwindow, tabwindow (picinpar.sty)
SCfigure, SCtable (sidecap.sty)
sidewaysfigure, sidewaystable (rotating.sty)
subfigure, subfigure*, the \subfigure macro
(subfigure.sty)
supertabular (supertab.sty)
wrapfigure (wrapfig.sty)
If you want to use other labeled environments, defined with
\newtheorem, RefTeX needs to be configured to recognize
them (see Defining Label Environments).