Objection 1. It would seem that general suffrages avail those for whom special suffrages are not offered, as much as special suffrages avail those for whom they are offered in addition to general suffrages. For in the life to come each one will be rewarded according to his merits. Now a person for whom no suffrages are offered merited to be assisted after death as much as one for whom special suffrages are offered. Therefore the former will be assisted by general suffrages as much as the latter by special and general suffrages.
Objection 3. Further, the Eucharist is the chief of the suffrages of the Church. Now the Eucharist, since it contains Christ whole, has infinite efficacy so to speak. Therefore one offering of the Eucharist for all in general is of sufficient value to release all who are in purgatory: and consequently general suffrages alone afford as much assistance as special and general suffrages together.
On the contrary, Two goods are more eligible than one. Therefore special suffrages, together with general suffrages, are more profitable to the person for whom they are offered than general suffrages alone.
I answer that, The reply to this question depends on that which is given to the twelfth inquiry (12): for if the suffrages offered for one person in particular avail indifferently for all, then all suffrages are common; and consequently one for whom the special suffrages are not offered will be assisted as much as the one for whom they are offered, if he be equally worthy. On the other hand, if the suffrages offered for a person do not profit all indifferently, but those chiefly for whom they are offered, then there is no doubt that general and special suffrages together avail a person more than general suffrages alone. Hence the Master, in the text (Sent. iv, D, 45), mentions two opinions: one, when he says that a rich man derives from general, together with special suffrages, an equal profit to that which a poor man derives from special suffrages alone; for although the one receives assistance from more sources than the other, he does not receive a greater assistance: the other opinion he mentions when he says that a person for whom special suffrages are offered obtains a more speedy but not a more complete release, because each will be finally released from all punishment.
Reply to Objection 1. As stated above (12, ad 2) the assistance derived from suffrages is not directly and simply an object of merit, but conditionally as it were: hence the argument does not prove.
Reply to Objection 2. Although the power of Christ Who is contained in the Sacrament of the Eucharist is infinite, yet there is a definite effect to which that sacrament is directed. Hence it does not follow that the whole punishment of those who are in purgatory is expiated by one sacrifice of the altar: even so, by the one sacrifice which a man offers, he is not released from the whole satisfaction due for his sins, wherefore sometimes several Masses are enjoined in satisfaction for one sin. Nevertheless, if any thing from special suffrages be left over for those for whom they are offered (for instance if they need them not) we may well believe that by God's mercy this is granted to others for whom those suffrages are not offered, if they need them: as affirmed by Damascene (Serm.: De his qui in fide dormierunt) who says: "Truly God, forasmuch as He is just will adapt ability to the disabled, and will arrange for an exchange of deficiencies": and this exchange is effected when what is lacking to one is supplied by another.
Source: Summa Theologica (New Advent)