Codex Astartes

The Codex Astartes is a book written by Roboute Guilliman that was adopted by the Space Marines after the Horus Heresy had ended. The book contains vast quantities of information on battle stratagems, army organization, and other useful information. However, the main requirement of the Codex was a demand that all space marine legions had to be broken up into chapters of no more than 1000 men, in an effort to prevent a repeat of the Horus Heresy. In TTS, The Emperor openly despises the breakup of the legions and hates Guilliman for writing the Codex Astartes.but later accepted it as a good idea for the sake of the imperium

Not all chapters follow the Codex in its entirety:
 * The Ultramarines follow the Codex Astartes to the letter, to the point that any battle brother who deviates from it is a potential heretic. Uriel Ventris nearly got punished for such a crime, whereas Cato Sicarius strictly enforces it.
 * Most chapters follow the Codex Astartes with minor changes specific to their chapter, such as the Imperial Fists, White Scars, Iron Hands, Raven Guard, and Salamanders.
 * The Blood Angels, Dark Angels, and their successors have major changes to their organizational structure for varying reasons, from the Blood Angels’ Death Company for afflicted battle brothers, to the Dark Angels’ reserving of the Deathwing and Ravenwing companies for the inner circle.
 * The Space Wolves and Black Templars reject the Codex in its entirety. However, the Space Wolves are unable to rebuild to legion strength due to genetic defect, and the Black Templars keep their forces scattered to hide their true numbers.
 * The Deathwatch and Grey Knights are largely exempt due to being specialist organizations working under the Inquisition.