The Arahat Athersata is also the first level (or plane) where dwell the pure spirit forms. It takes a spirit 60 to 80 billion years to evolve into this pure spirit form.
It is also the name of a book transcribed by Billy Meier that contains advice to the Earth human from pure spirit forms on the Arahat Athersata plane.
Quote from Contact Report 3
39. A human being may become hundreds or even thousands of years old when he has reached a certain spiritual and consciousness-related state of relative perfection and a correspondingly high and sound way of living.
40. But after that state of about 40 to 60 million years, the spirit no longer requires a purely material body and becomes a half-spirit body.
41. It then lives only in half-material form for an additional 60 to 80 billion years and in such lofty spheres that we can no longer communicate with it from the purely material human level, except when there exists a very high consciousness-related and spiritual level of evolution.
42. After 60 to 80 billion years, the half-spirit form—the half-material form respectively—transforms itself into the first pure spirit form and enters the level of Arahat Athersata.
See Arahat Athersata (book) for more.