Meaning of the symbol “$\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$” One of the back exercises (13(C)) from the book for practice provided at my school asked whether the following relation is true:

$\Delta_\mathrm{f}H^\circ(\ce{I2,g}) = \Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ at $\pu{25 °C}.$

I know how to solve the question, but the answer would depend on the meaning of  $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}].$ Which of the following reaction does the symbol $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ represent?
$$
\begin{align}
\ce{I2(g) &amp;-&gt; I2(s)}\tag{a}\\
\ce{I2(s) &amp;-&gt; I2(g)}\tag{b}
\end{align}
$$
$\ ΔH_{sub}(I_2,s)$ can be interpreted as the change in enthalpy when one mole of solid $\ I_2$  converts to gaseous $\ I_2$  at a constant temperature. So the answer should be reaction (b).