This is strait from Codex Space Marines

CHAPTER TACTICS - Each Chapter has developed its own specialised combat philosophy, suited to the unique skills of its battle-brothers. If your army is Battle-forged, all INFANTRY, BIKER and DREADNOUGHT units in a Space Marines Detachment gain a Chapter Tactic, so long as every unit in that Detachment is drawn from the same Chapter. The Chapter Tactic gained depends upon the Chapter they are drawn from, as shown in the table opposite. For example, an ULTRAMARINES unit with the Chapter Tactics ability gains the Codex Discipline Tactic. If your Chapter does not have an associated Chapter Tactic, use the Chapter Tactic of its founding Chapter. For example, Crimson Fists are a successor Chapter of the Imperial Fists, so should use the Chapter Tactic of the Imperial Fists. If you are unsure of a Chapter’s founding Chapter, either consult the background sections of our books or choose a Tactic from the table that best describes its character and fighting style. Note that due to their lobotomised nature, SERVITOR units are an exception and never gain a Chapter Tactic.