Some of the Control Brains also reside on Devastis evaluating an Irkens progress and can decide where to place the trainee in the Irken Army the highest rank to be placed is probably the Irken Elite where Irkens can train on to become the best class of Irken soldier; the Invaders.
Devastis is also home of the Irken Military Academy where present Almighty Tallest Red and Purple, as well as Zim used to attend when they were in the Irken Elite forces.
Devastis could be inside a nebula because the space around the planet is completely green like its sky. Devastis has no moons orbiting it. There also appears to be another planet somewhere nearby Devastis. In the past, half of the planet underwent a blackout when former Invader Zim used a Maimbot on a snack vending machine. This blackout caused testing on half of the planet to cease. Tak's Elite test was not completed, leading to her hating Zim.