More so than any other Space Marine chapter the Ultramarines adhere to the Codex Astartes, the doctrine governing the organisation, composition and tactics of the Space Marine chapters. As a founding chapter the Ultramarines bear considerable influence within the Imperium and are the model Chapter on which all others should be based. The XIII suffered the fewest causalities During the Civil War that resulted from the Horus Heresy, the XIII legion, from which the Ultramarines were formed, suffered fewer casualties than the other legions and was crucial in the role it played in the reconstruction of the post-Heresy Space Marine organization.
Because of the great influence and power the Chapter holds, the Ultramarines are in charge of an entire Ultramar Sector, which includes nine star systems and eleven worlds. Most Chapters control one home planet. The Ultramarines possess a large space fleet due to the size of their sector of influence. Since the Codex Astartes originated from the original Ultramarines and the fact that they have a vast number of accomplishments, they have a considerable amount of influence. Their numbers, however, are limited to the size of 1,000 Space Marines by the Codex.