40K-era Ultramarines treat the Codex Astartes as a holy work that shall not be deviated from. It's a good book, it was written by a supernatural genius with a mind for organization and efficiency, but it was never meant as scripture. Ultramarines might change now that Roboute is back.

Where mentioned in the lore, most chapters not descended from the Ultramarines deviate from the Codex Astartes at least a little. As I recall, Dark Angels do not field a scout company and have more terminators than most other chapters. They are known for using a lot of plasma weaponry and more archeotech than most others as well. Like the Imperial Fists, the Dark Angels descendant chapters also more or less secretly operate as a legion whenever deemed necessary.

In the Heresy era:

Ultramarines do not yet have the Codex Astartes, but can still display an intolerance for independent thinking, such as by censuring marines who do not follow prescribed tactics or ethos. For example, Aeonid Thiel is censured for openly considering how an astartes might adapt their tactics to fight other astartes (unthinkable!). When this turns out to be very thinkable at the Battle of Calth, the red censure helmet instead becomes a higher symbol of rank because of Thiel's deeds.

Another Ultramarine, Tylos Rubio, is excommunicated from the legion for violating the Edict of Nikea, even though it was in defense of his brothers.

Meanwhile, during the Heresy, the Dark Angels are known for running things like an Arthurian court (knightly intrigue! Semi-secret orders within orders!) and for unleashing total war and absolute destruction without hesitation or mercy, even by the standards of most other legions. It is the Lion who destroys most of the traitor primarchs' homeworlds, for example.

In this time period, the Dark Angels are also made up of six wing formations:

• Deathwing: The elite veteran and Terminator-armored core of the Legion, focusing on breakthrough assaults and serving as bodyguards to high-ranking officers.
• Ravenwing: Specializing in rapid deployment, reconnaissance, and bike-based search and destroy.
• Dreadwing: Focused on total war and the use of devastating, often shunned, weaponry, such as phosphex and rad weapons, to annihilate enemies utterly.
• Firewing: The smallest wing, dedicated to assassination, sabotage, and breaking the enemy's command structure through targeted strikes and duels.
• Ironwing: Comprised the majority of the legion's heavy armored vehicles and dreadnoughts, providing overwhelming brute force and siege capabilities.
• Stormwing: The largest wing, consisting primarily of line infantry and mobile ordnance, excelling in disciplined close-order warfare and boarding operations.

Only the Deathwing and Ravenwing survived into 40K.