Building a 40K army has changed quite a lot over the years, so I thought it might be worth jotting down my thoughts on putting together an army list. The main book tries to keep it simple, so it can be tweaked for each individual codex for each army, but that does leave it rather confusing too!
(Do take a look at my earlier post here which looks more at choice of army rather than the structure)
Bound and Unbound
First, there are two basic types of army. The first is called bound and essentially means that your army list is built entirely matching some of the various formations and detachments that are available (we’ll come to those in a moment). The second is called unbound and means, basically, that you are fielding whatever models you have without some of the normal combinations. You might have some detachments or formations in an unbound army (and may get their additional benefits too!) – it just means that you’ve broken some of the normal army building rules, and it’s great for narrative games.
Formations and Detachments
Well, these are actually pretty much terms for the same thing in practice. I think when the edition was first written, it was intended that “Detachment” would be an army list sized group, or close to it, while “Formation” was designed to be used for small platoon or even squad sized groups, but they are pretty much interchangeable now. I think technically detachment is applicable if there are bonuses that only apply if your army is “bound” while formations are self contained.
There are two detachments in the main book – the Combined Arms Detachment (which is the standard force org chart we’ve used over the last few editions of 40K), and the Allies Detachment (which is what we’ve used as standard for allies for a while too!). You have to take a HQ unit, and certain number of troops, and it puts restrictions on the number of each type of unit you can take. If everything in your army is part of a detachment or formation, then these detachments grant some extra bonuses – objective secured for the CAD, which means troops override other types if near objectives.
Early releases in 6th edition used those two detachments, and additional formations were sometimes provided in additional Dataslates. More recently, we’re seeing large numbers of formations and detachments within the codexes themselves – whether its unusual formations like the Harlequin and Necron codexes, or the simpler ones for Legion of the Damned or Codex Inquisition.
Where it gets a little more complex is some of the more advanced detachments are built out of smaller formations, and the bonuses stack. Codex Necrons has a detachment built entirely out of smaller formations, and it allows some really nasty sets of bonuses to be built up.
When written, I feel the original concept was to have a simple standard detachment for tournament games, and a slight advantage for following “traditional” patterns of armies, while allowing new players to field whatever they had, and for experienced players to try out those wacky fluffy concepts like dreadnaughts defending an abandoned fortress monastery from an work horde.
In practise, the ability to combine any number of detachments and formations means that you can generally build a legal “bound” army that gets you 99% of the way there. You can take a valid “bound” army of Eldar with all wraith warriors using a data slate, for example. There are Space wolf formations of dreadnaughts that could be used to get you towards a dreadnaught only army.
My recommendation would be that a starting player go with the combined arms detachment from the main book as a starting point. One HQ and 2 troops is a great starting point, I think, and it stops you overloading on one type of specialist troop like elites, fast attack or heavy support. Use unbound armies for custom games to tell stories, and get jazzy with formations once you have a better grip on the basics.