A "bound" army is any army that is composed just of detachments. There are many kinds of detachments, but any grouping of them is fine.
So, you could take an HQ and 2 Troops for a CAD, which gives your troops in that detachment Objective Secured.
1 HQ and 1 Troop is an Allied Detachment, which also has objective secured troops, but cannot be your Primary detachment. Note, the Allied Detachment is just a name - any other detachment can by from any faction, it's just the the Allied detachment has its own composition.
A Formation is a Formation Detachment, and you must take specifically what's in the formation. Formations often have special bonuses for the models/units in the detachments.
The Eldar (and Necrons) have a special detachment that's composed of smaller detachments. For the Eldar this is the Warhost. To make the Warhost you must take at least one of the three listed formations, and then take between 1 and 10 of the auxiliary detachments. Doing so lets those models gain the special rules for their formations, AND gain the special rules for the Warhost detachment.
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