What is Standard reaction quantity?

The standard reaction quantity $\Delta_r X^\circ(T) = \sum_i\nu_iX_{m,i}^\circ(T)$ is the reaction quantity computed with every species in its standard state at temperature $T$, the $X_{m,i}^\circ$ being the standard molar values. For $X = H$ it is the standard reaction enthalpy $\Delta_r H^\circ(T)$; it depends on $T$ only.