standard reaction quantities
Infinitesimal changes in thermodynamic functions with extent of reaction divided by the infinitesimal increase in the extent when all the reactants and products are in their standard states. For the quantity X they should be denoted by Δ r X°, but usually only Δ X° is used. For specific types of reactions the subscript r is replaced by: f for formation, c for combustion, a for atomization and superscript ‡ for activation.
Green Book, 2nd ed., p. 52 (