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I´m pretty sure I´ve read and/or heard somewhere that if a detachment includes models from different codicies/indices some rules may not be applicable, such as chapter tactics. If you for example use an Inquisitor as HQ in a Battalion Detachment with Militarum Tempestus Troops, their respective special rules would not apply, such as Orders for Militarum Tempestus. Have I just dreamt this...?

I believe its in the codex

Every Codex tells you the conditions to unlock their special army bonuses (Regiment bonuses, Chapter Tactics, Hive Fleet Adaptations, Inexorable Advance etc.) - usually that the Detachment must be 'pure', as people often shorthand it here. Some armies have exceptions to that, allowing other units without breaking the bonuses - again, each Codex will tell you.

Pages 132/133 of the Astra Militarum codex detail what abilities you gain for having pure detachments of specific regiments. You will lose regimental doctrines if you put an Inquisitor in to a detachment of Militarum Tempestus.

Orders are not such an ability though. An Astra Militarum officer can issue orders to valid targets regardless of what detachment they are in.

¨Thanks