What exactly is a Battleforged Army?
So despite the new edition having been out for a couple months now, I'm still shocked that people still seem to have no idea how to build an army in 7th edition! I'm only talking about Battleforged armies, as Unbound is just whatever you want.
So first up, lets talk general construction, and because I'm a designer, I've made a handy little infographic:
So to begin with, what is that graphic telling us? First of all, you can make an army of pretty much any combination of factions that you like, though they'll treat each other's units as allies as per the allies section of the rulebook. That means you could have Space Marines with Imperial Guard with Chaos Daemons. The Space Marines and Guard will treat each other as Battle Brothers, but will both treat the Chaos Daemons as Come the Apocalypse.
Also, just for clarity, Formations are a special type of Detachment, but are a Detachment none the less.
Your Primary Detachment is simply whatever Detachment contains your Warlord. If your Warlord is in a Formation, congratulations, that Formation is your Primary Detachment (and thus cannot have an Allied Detachment of the same faction, though may use other types of Detachments of that same faction).
Your Primary Detachment is not necessarily your Combined Arms Detachment, though there is a bonus if your Warlord indeed comes from a Combined Arms Detachment.
Detachment Bonuses and Restrictions
Different Detachments have different bonuses and restrictions. These bonuses and restrictions generally only apply to the models within that detachment (any of the darker grey circles above is a Detachment). This means if you take a Combined Arms Detachment and a Formation, the Troops in the Combined Arms Detachment will benefit from Objective Secured, while any Troops in the Formation will not.
When there were rumors of 7th edition, people got the idea that ALL Troops in a Battleforged army were Objective Secured. That is just plain incorrect. Only troops that are given the Objective Secured rule through the Faction Bonus (or some other means) will be Objective Secured, Troops are NOT intrinsically Objective Secured.
Your Primary Detachment can be ANY detachment other than an Allied Detachment. It can be from a Formation, Combined Arms, or any other faction specific detachment type.
Your Allied Detachment may NOT be the same faction as your PRIMARY Detachment. That means if your Warlord comes from a Formation (Chaos Space Marines for example), your army may also contain a Combined Arms detachment of Chaos Space Marines, but it may not contain any Allied Detachments of the Chaos Space Marines faction.
An Unbound army is simply an army where you make an army where you can take any models you like and the only Detachments allowed are Formations.