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The Holy Souls are not present in the same manner as Our Lady and the Saints (unless as a special grace from God granted to them), but they do benefit immensely from the Holy Sacrifice of the Mass, especially Masses offered particularly for them. I think the above image does a good job of capturing what is occurring in the spiritual realm during the Mass. Notice the angel pouring the Precious Blood out over the souls in purgatory, who, though the Mass and the glory of heaven are veiled from them, benefit greatly from its effects.