12 Besides all this, the grace of mutual charity among the living, which derives from the Sacrament of the Eucharist so great an increase of strength, is further extended by virtue of the Sacrifice to all those who are numbered in the Communion of Saints. For the Communion of Saints, as everyone knows, is nothing but the mutual communication of help, expiation, prayers, blessings, among all the faithful, who, whether they have already attained to the heavenly country, or are detained in the purgatorial fire, or are yet exiles here on earth, all enjoy the common franchise of that city whereof Christ is the head, and the constitution is charity. For faith teaches us, that although the venerable Sacrifice may be lawfully offered to God alone, yet it may be celebrated in honour of the saints reigning in heaven with God Who has crowned them, in order that we may gain for ourselves their patronage. And it may also be offered - in accordance with an apostolic tradition - for the purpose of expiating the sins of those of the brethren who, having died in the Lord, have not yet fully paid the penalty of their transgressions.
Source: Mirae Caritatis (Vatican.va)