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Does taking a captain/librarian/chaplain in one force organization unlock it for an ally? To be more specific If I take Tigurius in my UM main force and a Chapter Master of IF in my ally can I take an IF command squad or does it have to a UM command squad?

Doesn't the command squad not take up an FOC slot?

You are correct it doesn't take up a spot. But the issue is being able to take a command squad for IF even though I unlocking it with the UM character. Unfortunately Chapter masters don't unlock command squads.

IIRC detachments are detachments so if unit a in detachment a unlocks unit b then unit b belongs to detachment a

By RAW you can do this but by RAW you can take a command squad for an Allied BA Captain or DA Librarian, or indeed take a techmarine for an allied Ork Warboss or Necron Overlord. This issue is the term "army" is defined in the BrB but is frequently used in a way that is supposed to mean detachment essentially acting in ignorance of the allies rules.

By the same reasoning I can take Ork fast attack choices for my space marine primary detachment because an army is an army...

By the same reasoning I can take Ork fast attack choices for my space marine primary detachment because an army is an army...

Sorry but this I do not understand how can you take Ork FA slots in a SM detachment?

The Techmarine rules say for each HQ choice in your army you may take one (with some exceptions). By army the BRB defines us to mean all Primary and Allied detachments. So we can have a Techmarine for each HQ choice in our primary or allied detachments...

One of the many reasons blindly following RAW is dumb.

 FlingitNow wrote:
By army the BRB defines us to mean all Primary and Allied detachments.

Careful with that statement. You'll open a huge can of worms.

 d-usa wrote:
By the same reasoning I can take Ork fast attack choices for my space marine primary detachment because an army is an army...

All choices for all primary detachments must come from the same codex. All choices for al allied detachments must come from the same codex that is not the same codex as the primary detachment. There are no ork fast attack units in your space marine codex.

With Tigurius in either detachment the requirements are met for either detachment to take a total of 1 command squad (assuming both detachments have the command squad unit option) because Tigurius is in your "army".

Usually the space marine book is more careful with the use of 'detachment' and 'army' in the rules because it can ally with itself. For example look at mounted assault and abhor the witch. Both use the word detachment where another codex may have used army. Does this mean the command squad use of army was intentional? Only an errata could tell us for sure but its not how I will play it.