The Imperium classifies psykers on the "Assignment" scale, ranging from Omega (virtually no psychic potential) to Alpha Plus (planet-destroying potential).
Immensely powerful psykers capable of destroying battleships single-handedly or even controlling entire planets. Usually unstable and extremely dangerous. Examples include the Emperor and primarchs.
Extremely powerful psykers who can affect entire battlefields. Chief Librarians and the most powerful Farseers may reach this level.
Very powerful psykers that include most Space Marine Librarians, Eldar Warlocks, and powerful Astropaths.
Moderately powerful psykers that include Imperial Sanctioned Psykers and Astropaths.
Mild psykers, often used as Astropaths or for other specialized Imperial services.
The psychic warriors of the Adeptus Astartes. They combine their gene-enhanced physical might with devastating psychic abilities, serving as both warriors and repositories of chapter knowledge.
Blind psykers who specialize in telepathic communication across the vast distances of space. The backbone of Imperial communication.
Psykers who serve with the Imperial Guard, providing psychic support to military operations.
Psykers who serve the Inquisition, using their powers to detect heresy, interrogate suspects, and combat daemonic threats.
Elite combat psykers who serve in the Astra Militarum, capable of unleashing devastating psychic attacks against enemy forces.