[MidnightSun]

A Combined Arms Detachment contains 1-2 HQ, 0-3 Elites, 2-6 Troops, 0-3 Fast Attack and 0-3 Heavy Support, all drawn from the same Faction. The BRB defines Faction on page 118 as:

Adepta Sororitas
Astra Militarum
Blood Angels
Chaos Daemons
Chaos Space Marines
Dark Angels
Dark Eldar
Eldar
Grey Knights
Imperial Knights
Inquisition
Necrons
Orks
Space Marines
Space Wolves
Tau Empire
Tyranids

As the various Supplements draw from a 'parent' Codex, they share that army's Faction. So Eldar includes Iyanden, Chaos Space Marines includes Crimson Slaughter and Black Legion, Tau Empire includes Farsight Enclaves and Orks contains Waaagh! Ghazghkull.

Your army consists of a number of Detachments and/or Formations. You can have as many Detachments as you wish, of any kind. Most Detachments state that all units within the Detachment consist of units from the same Faction (the Combined Arms Detachment and Allied Detachment both have a clause to this effect).

There are a few restrictions on Allied Detachments due to the outdating of 6th edition wording by 7th edition army composition - Iyanden, Waaagh! Ghazghkull and (unconfirmed) Heroes of Fenris cannot ally with their parent Codex. All other Supplements are free to do so. There is a workaround, in that you can still bring a Combined Arms Detachment of Iyanden alongside your Combined Arms Detachment of Eldar.

 

[MidnightSun]

This is the crux of the debate right here; can you quote anything that would explain why 'All other Supplements are free to do so'? Are we still referring to the rule that says [Supplement] can Ally as Battle Brothers with [Codex]? Because the whole point of this argument was what that rule means. If it's a matter of Codex trumping BRB, don't all of those supplements also state that they count as their parent's codex, forcing the 'Same Faction' rules for Allied Detachments right back into play?

Ah, I see - then RAW, no, you can't bring an Allied Detachment of your codex' supplement because Allied Detachments cannot have the same Faction as your Primary Detachment.

But you can bring a Combined Arms Detachment or any of the race-specific Detachments or Formations, I don't know why it would be an issue for RAW players. If you're not a silly RAW player, then it's fairly obvious from the Farsight Enclaves book saying 'You can ally with Tau' and Iyanden saying 'You can't ally with Eldar' meaning just that.

 

[MidnightSun]

What if you went:

CAD (Primary) Tau
CAD Eldar
AD Iyanden

where you are no longer in violation of the clause that prohibits Allied Detachments from being the same Faction as the Primary Detachment?

Or is there a statement from Iyanden supplement that specifically states something further on the way it can work with the Eldar Codex?

I can't remember the wording off-hand, but I know Iyanden and Waaagh Ghazghkull are unusual in that you can't ally them with the parent book. Iyanden is out of date, so I'm guessing the multiple CAD thing works round that, but Ghazghkull's a 7th ed book so I'll have a look at that when I can.

I think the idea you posted would work with a Tau Primary, but I'd wonder why you wouldn't just bring Combined Detachments instead of messing around trying to get them into an Allied detachment. To be honest, I don't think much of Allied Detachments unless you want one very specific unit from a Codex (looking at you, Space Wolf Drop Pods, and you Grav Centurions) - why limit yourself to 1 slot of each when you could bring an extra Troop and get 3 slots for each Battlefield Role instead?

That said, I think it's against the spirit of the rules and I'd be very leery of it. Iyanden says you can't ally and back then that was the only way to combine them with Eldar - it's simply a case of old rules not functioning in a new edition. Working around it with an obvious loophole is pretty poor, in my view, unless you had one absolutely amazingly cool thing you liked that you couldn't do without combining the books.

 

[MidnightSun]

You can bring a bunch of Chapter Tactics in one army - you just need to bring a bunch of Troops and HQ to get more Combined Arms Detachments, and basic SM Troops aren't particularly strong outside of Bikers (at which point you need to bring multiple Captains on Bikes, which is very expensive).

So yes, rather limited to big games

 

[MidnightSun]

Yeah, the 'I have 60 Tacticals, deal with them' is one of those lists that's really good against some lists and really bad against others. I'd be interested to know what kind of firepower it puts out - Tacticals are poor, offensively, so presumably there's support of some kind? Difficult to balance the different firepower you need with just MEQ bodies.

EDIT: For anyone going back and reading this thread again, I realize that putting 60 or 70 dudes in Drop Pods and forcing the enemy to deal with them actually does work, since two of the BAO 2014 top ten lists were basically that (and both had Calgar, flying in the face of what is 'competitive'.

 

[MidnightSun]

I think the issue with Iyanden is actually handled by the Faction rules. As Iyanden counts as Eldar, they count as the Eldar faction... one of the reasons why you can count them as Battle Brothers on the matrix, but can't take them as an Allied. Iyanden didn't add this rule in their codex, which was an oversight, but the Faction rules remove the need for it... which might explain why you don't need it in the Ork Waaaawhatever (I don't know Orks, sorry).

Simply put, any supplement is now the same faction as the core race. You can always CAD them, but you can never Ally them. With the exception being Marines, because they treat each Chapter as its own faction in terms of using an Allied Detachment.

That makes a lot of sense, yeah. :victory:

 

[MidnightSun]

3 Devastator squads do a pretty good job of supporting. Not to mention the Special/Heavy Weapon combo in each Tactical squad. It's obscene.

Fair enough, hadn't considered Devastators in truth - I'd definitely want to face that across a field and see if I could deal with it!