In Wh40k Space Marines get several Special Rules (called USR = Universal Special Rules).
The most well known is
"And They Should Know No Fear"
Allows Space Marines to automatically regroup. This works even if their squad is below half strength (non-ATSKNF units can't regroup if below half strength).

"Combat Tactics"
Every non-Fearless unit can choose to automatically fail a Leadership test.

All Codex Chapters use "Combat Tactics"

Now we come to "Chapter Tactics". "Chapter Tactics" replace "Combat Tactics" with a new rule. "Chapter Tactics" are usually active if you include some certain Character (often this is the Chapter Master of a particular Capter) in your army.

Here are the current "Chapter Tactics" from various Codexes and ImperialArmour books:

Crimson Fists (Chapter Master Pedro Kantor): Exchanges "Combat Tactics" for "Stubborn". Enables Sternguard Veterans to be scoring units.

Imperial Fists (Captain Lysander): Exchanges "Combat Tactics" for "Stubborn".

Raven Guard (Captain Shrike): Exchanges "Combat Tactics" for "Fleet of Foot".

Salamanders (Forgefather Vulkan He'stand): Removes "Combat Tactics" but Flamers, Heavy Flamers, Meltaguns and Multi-meltas are counted as twin-linked and Thunderhammers are counted as master-crafted.

White Scars (Captain Kor'sarro Khan): Exchanges "Combat Tactics" for "Flank March" even for Dedicated Transports.

Blood Angels: Exchanges "Combat Tactics" for "The Red Thirst" (roll a D6 for every unit with this rule. On a 1 they gain "Furious Charge" and exchange ATSKNF for "Fearless")

Space Wolves: Exchanges "Combat Tactics" for "Counter Charge" and "Acute Senses".

Red Scorpions (Lord High Commander Carab Culln): Removes "Combat Tactics" but every Veteran Sergeant in Tactical Squads can be upgraded to an Apothecary (which gives the Squad "Feel no Pain").

Fire Angels (Captain Tanrus Vane): Exchanges "Combat Tactics" for "Tank Hunter". Any Rhino and Razorback gains Extra Armour.

Raptors (Chapter Master Lias Issodon): Exchanges "Combat Tactics" for "Stealth".

Fire Hawks (Knight-Captain Elam Courbray): Removes "Combat Tactics" but every Flamer and Heavy Flamer counts as twin-linked. Also every Assault and Vanguard Assault Squad counts as scoring unit.

Mantis Warriors (Chief Librarian Ahazra Redth): Exchanges "Combat Tactics" for "Infiltrators" (except for Terminators and untis with Dedicated Transports).

Executioners (Lord High Chaplain Thulsa Kane): Exchanges "Combat Tactics" for "Stubborn".

Minotaurs (Lord Asterion Moloc): Exchanges "Combat Tactics" for "Prefered Enemy: Space Marines".

Carcharodons (Tyberos The Red Wake): Exchanges "Combat Tactics" for "Furious Charge". Also gains "Rage" if they win any hand-to-hand-combat with an infantry unit.

Sons of Medusa (Iron Thane Vaylund Cal): Exchanges "Combat Tactics" for "Fearless".

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And in case you don't know what all of those USR do here is an explanation rundown:

Stubborn = Ledership tests are made without any negative modifiers. Very importand in hand-to-hand combat as the loosing side has to pass a Ld-test to prevent a withdrawal. This Ld-test is usually modified by -1 for every casualty. Stubborn units ignore this modifier.

Fleet of Foot = A Unit with this can "Run" in the Shooting Phase instead of shooting (as every Infantry unit can) but is still able to charge an enemy unit in the Assault Phase.

Flank March = Normally if units arrive from reserve on the board they arrive from the owning players board edge. With Flank March the unit can arrive from the side edges too.

Counter Charge = A unit which is charged gains +1 Attacks as if they had charged.

Acute Senses = Unit can re-roll how far they can see in night fight scenarios.

Feel no Pain = After a model has lost a wound (almost all standard units haveonly 1 wound) you rall a D6. On a 4+ it is unharmed. Can't be used against what is considered to be a maco-weapon in Epic.

Tank Hunter = Add +1 on armour penetration rolls (shooting and hth-combat).

Stealth = Unit gains +1 on Cover Saves.

Infiltrators = After deployment of both armies Infiltrators can be deployed anywhwere on the table but not closer than 12" from enemy units if the enemy unit can't see them or 18" if they can be seen.

Prefered Enemy = Against the stated prefered enemy to-hit rolls in hth-combat can be re-rolled.

Furious Charge = On the charge the unit gains +1 Weapon Skill (used for to-hit rolls in hth-combat and +1 Initiative (the models with the highest Initiative strike first)

Rage = Unit is forced to always move toward the nearest enemy unit they can see. In the Shoting Phase they may shoose to run bit this too towards the nearest visible enemy unit. If possible they have tocharge the nearest visible unit in the Assault Phase.

Fearless = Unit always pass any Leadership or Pinning tests.

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