It was answered elsewhere on the forums, I believe, but I am interested in hearing it again in detail because it is possible to pick up many partial indulgences for the living and deceased during the course of one day, but only one plenary indulgence per day. (If I understand correctly)
Sort of like “You can have one complete, untorn paper dollar bill per day to completely pay a dollar store merchant for a vase your individual child has broken when you ask so your child does not have to sweep his floor for four weeks.” (a silly look at the Plenary indulgence) The partial indulgence (where only part of the sin is remitted) would be like getting coins up to 99 cents for one individual, sort of like “You can have 99 cents so your child will only sweep the floor for one day.”
Why split hairs? What if you have a child that has already paid by sweeping the floor for one day? Then your partial indulgence may be enough to get them out–thus the St. Gertrude Prayer for the souls in Purgatory help “A great number” (traditionally held to be 1000, but changed to “A great number” so as not to limit God’s generosity. Thus, a partial could act like a plenary – both have removed sin – but for getting out a specific soul (Since the St. Gertrude Prayer does not specify WHICH children you may be rescuing from sweeping the floor – just that there were at least 1000 broken vases

you would want to go with the plenary if you could.
But, of course, any child rescued from floor sweeping will be eternally grateful!
Yours in Christ