The Divine Mercy Indulgence and the Divine Mercy Promise are two completely separate things.
The Divine Mercy
Plenary Indulgence granted by the Church (not by Jesus, but by the Church) has the usual conditions attached to it: do the indulgenced work (certain devotions on Divine Mercy Sunday), be in state of grace when you do the indulgenced work, no attachment to sin, confession within 20 days before or after, be in a state of grace, Holy Communion within a few days, and prayer for Holy Father’s intentions.
Like all indulgences, you can keep it for yourself or give it to a poor soul in Purgatory.
The Divine Mercy
Promise is
- part of Approved Private Revelation, so while the Church says it’s worthy of belief, Catholics don’t have to believe it
- granted directly by Jesus, NOT by the Church
- only requires reception of Holy Communion on that day, in a state of grace, with trust in The Divine Mercy
- And also, you cannot give these graces to any other soul; you get them from Jesus, for you and you alone.
Note that the Divine Mercy Promise does NOT require you to do a devotion, pray for the Holy Father’s Intentions, or most importantly, have no attachment to sin. It just requires you to trust in the Divine Mercy.
The Church indulgence is NOT repeat NOT some attempt to endorse the Divine Mercy Promise by writing it into indulgence form. (Like Jesus really needs the Church to okay him giving somebody grace…not.) Indulgences are given to encourage the faithful to do certain activities; in this case it was clearly given to promote the Divine Mercy Devotion among the faithful.
Here is a more detailed article explaining the difference between the Indulgence and the Promise.
I usually try to get both on Divine Mercy Sunday.
I keep the graces of the Promise for myself. I give the indulgence away to a poor soul in Purgatory.