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After a number of posts from new or returning players confused by the convoluted nature of detachments and bound armies in 40k I've come to the conclusion that a related problem is the lack of clear/consistent terms. I'd like to propose the following convention, see it it'll make things easier.

Detachment: The basic unit of a bound army. A collection of units assembled according to certain restrictions.

Three types of detachments:
Meta-detachment: A detachment composed of a number of formations. Decurion, Warhost, Gladius Strike Force, etc.
Formation: A detachment chosen from a constrained set of specifically defined units. Librarius Conclave, Aspect Shrine, Ironstrider Cavaliers, etc.
FOC (Force Organization Chart): A detachment chosen from a broad selection of units according to their combat role type (HQ, Troops, etc). Combined Arms Detachment, Allied Detachment, Skitarii Maniple, Inquisitorial Detachment, etc.