When choosing an army, all models in the same Detachment or Formation must be drawn from the same Chapter. If a unit's datasheet contains a Chapter in brackets after the Chapter Tactics special rule - for example, "Chapter Tactics (Ultramarines)" - the unit must always be drawn from this Chapter, and therefore cannot be included in a Detachment or Formation with any units that are drawn from other Chapters. All models drawn from a given Chapter benefit from that Chapter's Special Rule. If a unit contains models drawn from two different Chapters, it counts as from neither Chapter, and thus benefits from neither Chapter Tactic.
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