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I'm a little unclear as to how detachments and formations work together, or if they do work together. Specifically, the Imperial Knight Oathsworn Formation. Now this formation can be 1-3 Knights, I get that, but if I have three Knights in the detachment can it also be a formation? If I put Icarus Autocannons on all of the Knights in the Detachment can it also then benifit from the Skyreaper Lance Formation? Another example, the Cult Mechanicus Battle Congration requires one HQ and two troops. If I have one Tech-Priest and two units of Breachers to make the minimum requirement does this also count as a Holy Requistioner Formation?

Thank you for your help.

I would highly recommend reading carefully the 'Detachment's section of 'Choosing Your Army' in the rulebook. Specifically to answer your question there are two bolded sentences partway through, one of which reads, in relation to Battle-forged armies, "no unit can belong to more than one Detachment."

The section later on Formations also tells us, "Formations are a special type of Detachment," and so we now know that ordinarily no unit can belong to more than one Detachment, which includes Formations.

There are exceptions, but these are generally explicitly stated to allow units to belong to more than one Detachment.

Quite helpful, thank you.

To piggy back on mrshine:the new "Decurion style" detachments are also special types of Detachment that have slots, have command benefits, and are comprised only of the specified Formations and units listed(with an allowance for the units to be part of both detachments: the formation and whatever the special detachment that formation is a part of).

 Kommissar Kel wrote:
To piggy back on mrshine:the new "Decurion style" detachments are also special types of Detachment that have slots, have command benefits, and are comprised only of the specified Formations and units listed(with an allowance for the units to be part of both detachments: the formation and whatever the special detachment that formation is a part of).

In addition to those, there are a few super formations which are made up of smaller formations that have a similar caveat as the Decurions allowing the units of the smaller Formations to keep the Special Rules of those Formation. Most came out before the current Necron codex, though. The biggest group of them are part of the Rising Leviathan series, I believe.

Even further into that is the Deathwatch stagium command or watch company: both are formations with a hero, and then a single choice from either a single unit or any of the other killteam formations. These 2 formations lack the caveat, but can be part of the Decurion-style detachment(which has the caveat); but we still aren't completely sure if we are just supposed to assume the caveat is in place, or if you can only use the formations listed in the composition when the formation is part of the blackspear strikeforce(the veterans squad as one of the options means the formations could be used with just hero and Veterans).