Honour the Codex
Honour the Codex is a Demeanour (see page 32) unique to Space Marines from the Ultramarines Chapter.
In all of their duties, from battle to study, the Ultramarines are thorough, analytical, and attentive to every detail. They are slow to anger and rarely make a rash decision. By constant recourse to the articles of faith enshrined within the Codex Astartes, the Ultramarines are able to face any eventuality with well practised battle drills and established doctrine.
In their dealings with Battle-Brothers from other Chapters, Ultramarines often fulfill the role of facilitator and peacemaker. While the Battle-Brothers of some Chapters display extremes of character, from hotheaded to taciturn, the Ultramarines are balanced in their approach, and often able to broker agreements between wildly differing points of view. This is evidenced in the fact that a high proportion of Deathwatch leaders are drawn from the Ultramarines and their successors, a fact that has more to do with their outlook and character than it does with their numerical superiority.
An Ultramarine is likely to turn to the vast tracts of the Codex Astartes that he has memorised for the solution to a problem.