Codex Astartes

The Codex Astartes was written by Roboute Guilliman shortly after the Horus Heresy; among other things, its suggested organizational scheme split the remaining Space Marine Legions into small, manageable chunks. It also includes tactical doctrines for just about every situation, like the formidable Steel Rain. Most loyalist Space Marines follow the Codex Astartes to varying degrees. The Ultramarines and their successors follow it like the holy writ, while the Space Wolves only find it useful for toilet paper.

Marine roles

Among the tenets of the Codex, Guilliman outlined the roles of various Marines to be used in Chapters. They include the follow:

• Assault: Assault Marines are those who specialize in melee combat, using Chainswords, Thunder hammers, and Power swords alike to fuck up the enemies of the Imperium of Man. In addition, they also possess jump packs which can allow them limited flight.

• Devastator: Devastator Marines can be summed up in three words: Blow shit up. The purpose of the Devastator is to provide heavy weapons fire in battle and hunting tanks.

• Scout: Scout Marines are newly initiated Space Marines who are charged with scouting terrain and enemy positions. A risky job, but it they're killed, the enemy probably did the Chapter a favor by killing them so quickly.

• Tactical: Tactical Marines are those who are flexible in combat, relying on both ranged weapons and melee weapons in combat.

• Veteran: Veteran Marines are those who have seen through several centuries, allowing them all the cool toys in the Chapter's armory.

Chapters

Under the Codex Astartes, the Space Marine legions were split into one thousand strong Chapters composed of ten companies. The first company is composed of Veterans, and are the only company in a chapter to employ Terminator armor.

The second, third, fourth, and fifth companies are composed called battle companies, composed of six Tactical Squads, 2 Devastator Squads, and two Assault Squads.

The sixth, seventh, eighth, and ninth companies are the reserve squads, and are more specialized than the first five companies. The sixth and seventh squad are composed entirely of Tactical Marines, the eighth company of Devastator Marines, and the ninth company entirely of Assault Marines.

The tenth company is composed entirely of Scout Marines, who are eventually promoted to other companies after a certain number of years.