[L]et us recall to mind that each of our good works, performed in the state of grace, ordinarily possesses a triple value in the sight of God.
- The work is meritorious, that is to say, it increases our merit; it gives us right to a new degree of glory in Heaven.
- It is impetratory, that is to say that, like a prayer, it has the virtue of obtaining some grace from God.
- It is satisfactory, that is to say that as having, as it were, a pecuniary value, it can satisfy Divine Justice and pay our debts of temporal punishment before God.
The merit is inalienable, and remains the property of the person who performs the action. On the contrary, the impetratory and satisfactory value can benefit others, in virtue of the communion of saints. —What are suffrages by which according to the doctrine of the Church, we may aid the souls in Purgatory?
To this question we answer: They consist of prayers, alms, fasts, and penances of any kind, indulgences, and above all the Holy Sacrifice of the Mass. All the works performed in the state of grace Jesus Christ allows for the relief of our brethren in Purgatory, and God applies them to those souls according to His Justice and Mercy.
—from Purgatory: Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J., Part Second: Purgatory, The Mystery of God’s Mercy – Ch. 9