Originally Posted by
Anthrax ion pusscabe
I've played 40k as blood Angels since the latter part of fifth and I can understand it when the core rulebook for 40k says and I quote "formations are a special type of detachment, each a specific grouping of units renowned for their effectiveness on the battlefields of the 41st millennium. Whilst some formations provide you with all the gaming information you will need to use them in your games, it is not uncommon for them simply to d scribe a number of special rules that apply when you include several specific units together. Instead of including a force organisation chart, the army list entries that comprise a formation are listed on it, along with any special rules that those units gain. unless stated otherwise, each individual unit maintains its normal battlefield role when taken as part of a formation
Unlike other detachments formations can be taken as part of unbound armys. If the are, their units maintain the special rules gained for being part of the formation" page 121
Then let's look at battle forged armys " a player using the battle-forged method must organise all the units they want to use into detachments. Detachments are made up to f units that conform to various requirements. For example, one common type of detachment requires the use of at least one HQ unit and two troops under ts; another might require that only units from codex orks be included. As a reward for adhering to these requirements, each detachment grants it's own command benefits to the units within it, which can really enhance their effectiveness in battle" page 117
A battleforged army therefore is made of detachments and formations are a type of detachment, so therefore a army can include formations and remain battleforged, though reading the combined arms detachment it does say
"Objective secured: all troops units from this detachment have the objective secured special rule. A unit with this special rule controls objectives even if an enemy scoring unit t is within range of the objective marker, unless the enemy unit has this special rule" page 122
So we are both semi right, I am right in formations being a type of detachment instead of the other way around but I am wrong in thinking it's being a battle forged army that grants objective secured instead of the combined arms detachment