\startsubformulas[eq:total]
\placeformula
\startformula
Formula \eqref[eq:total] states the Pythagora's Theorem twice,
once in \eqref[eq:first] and again in \eqref[eq:second].
You can compare without \tex{startsubformulas}:
\placeformula
\startformula
\startalign
\NC a^2 + b^2 \NC = c^2 \NR[eq2:first]
\NC a^2 + b^2 \NC = c^2 \NR[eq2:second]
\stopalign
\stopformula
We now have references for \eqref[eq2:first] and \eqref[eq2:second].
</context>