The elevations of the souls from the lower Gan Eden to the higher Gan Eden is via the [intermediary of] Nahar Dinur (river of fire) which is the pillar in Gan Eden which includes the concept of self nullification (bitul), [that one must have bitul] in order to come from the lower Gan Eden to the higher.
So, too, in the days of Moshiach there will be a resurrection of the dead as an intermediary level until the World to Come in the 7th millenium. The difference between immersing in the Nahar Dinur or becoming nullified in the pillar [on the one hand] and the level of intermediary in the days of Moshiach which refers to the resurrection of the dead, is that the immersion in the Nahar Dinur is nullification for the soul alone and not the physical bodies. But the resurrection that is after the days of Moshiach is the aspect of nullification for the bodies as well, meaning that the physical bodies themselves will grasp the Divine light and they will be on the level of self-nullification of their very existence.
Maamorim of the Alter Rebbe, 5565, I:412