Tanya Chapter 39 part 6

The world of Atzilus is beyond the intellect of a created being, therefore it is the abode of the sublime souls who served Hashem in a way that completely transcends the intellect. As a chariot is nullified to the rider, these great tzadikim are completely surrendered to the Divine will–– and they, as well as everything that they own, expresses nothing other than the will of Hashem. People who are not capable of this level of surrender because their soul’s root lacks this capacity, have the opportunity to reach this level at times of Divine favor, such as during the amidah prayer. And even though they do occasionally experience the nullification of a chariot, the principal abode of their soul is not in the world of Atzilus. Instead, it is in the world of Beriah, and only on special occasions can it take the opportunity to rise to the world of Atzilus.