Successfully complete the indicated challenges to complete the codex entries. The codex entries unlock new multiplayer modifications, like weapon mods. If you want to unlock everything in Multiplayer mode, you must complete all of the challenges and find all 66 Field Manuals. The following is a list of all the codex entries in the campaign:
War Story 1: Through Mud And Blood
Mission 1: Over The Top
German Anti-Tank Tactics: Destroy all field guns.
Observation Balloons: Defeat five enemies while on foot.
Tank Shock: Collect all Field Manuals.
Mission 2: Fog Of War
Guiding Bess: Get through the forest without being detected.
Forest Combat: Get through the forest without any melee kills.
Gone Chemical: Collect all Field Manuals.
Mission 3: Breakdown
Stealing Tanks: Retrieve all engine parts undetected.
Tank Hunters: Retrieve the first engine component.
Aircraft Support: Collect all Field Manuals.
Mission 4: Steel On Steel
Bourlon Wood: Do not let the tank get damaged before clearing the village.
Beutepanzers: Destroy one of the captured MK V tanks.
There's Always a First: Collect all Field Manuals.
Towards Cambrai: Complete "Through Mud And Blood".
War Story 2: Friends In High Places
Mission 1: Test Flight
To Be Ace: Finish the target practice.
Dicta Boelcke: Do not lose the trail during the chase.
Mission 2: Total War
Bloody April: Have at least four bombers survive a bombing run.
Strategic Bombing: Destroy the fort.
Mission 3: Fall From Grace
No Man's Land: Reach no man's land undetected.
Booby Traps: Use tripwire bombs to kill two enemies.
A War in France: Collect all Field Manuals
Mission 4: Forte Et Fidele
Gotha Raids on London: Take down the bombers.
Airborne Cannons: Destroy 10 aircraft within 30 seconds using the stationary weapon.
Jousting with Dragons: Collect all Field Manuals.
Dawn of Dogfights: Complete "Friends In High Places".
War Story 3: Avanti Savoia!
Mission 1: O La Vittoria
Farina Armor: Defeat five enemies with grenades.
The Daring Ones: Join the fight as an Arditi Elite Troop.
Coming Together: Collect all Field Manuals.
Mission 2: O Tutti Accoppati
Where Eagles Dare: Defeat ten enemies with your sidearm.
Beyond Belief: Witness the explosion of the mountain side.
Alpine War: Collect all Field Manuals.
Among Peaks of Kings: Complete "Avanti Savoia!"
War Story 4: The Runner
Mission 1: Cape Helles
The Big Guns: Call in an offshore barrage.
Torpedo Exploit: Get ten headshot kills.
Collect all Field Manuals.
Mission 2: The Runner
Drip Gun: Defeat ten enemies with a sidearm.
Geology: Defeat five enemies with rifle grenades.
Trench Runners: Collect all Field Manuals.
Mission 3: Be Safe
River Clyde: Deafeat five enemies with grenades.
Sedd El Bahr: Defeat ten enemies with melee attacks.
Gallipoli Myths: Collect all Field Manuals.
The Dardanelles: Complete "The Runner".
War Story 5: Nothing Is Written
Mission 1: Hidden In Plain Sight
The Hejaz Railway: Scout the crash site.
The Bedouin: Find the book without being detected.
Celebrities: Collect all Field Manuals.
Mission 2: Young Men's Work
Gertrude Bell: Kill one of the officers by using a bolt-action rifle.
T.E. Lawrence: Kill one of the officers with a melee attack.
Tanks in the Desert: Kill one of the officers by using a tank.
Women Rise: Collect all Field Manuals.
Mission 3: Hear The Desert
Desert Cars: Destroy the three vehicles while undetected.
Planes Above the Sand: Keep at least two field guns operational.
Guerilla Warfare: Collect all Field Manuals.
Arab Revolt: Complete "Nothing Is Written".