***Update: The full Ob Sec faction list can be found here: https://www.frontlinegaming.org/2017/08/15/itc-obsec-factoin-keyword-list/
At 1st read it is easy to overlook the 1st sentence. If your army is Battle-forged, all Troops units in <Faction> Detachments gain this ability.
<Faction> Detachment only applies to specific keywords, especially those that replace <chapter> or <legion>. A list of which faction keywords apply to Ob Sec will be coming from the ITC shortly.
But as a general rule of thumb, you can use this logic:
- Very broad faction keywords don't get you ob sec. For instance "Imperium", "Chaos", or "Aeldari"
- Very specific faction keywords do get you ob sec. For instance "Ravengaurd", or "Ultramarines"
- Some medium specificity keywords do get ob sec. For instance "Ad Mech" or "Astra Militarum"
- In Order to qualify for Ob sec every model in a detachment must share those faction keywords.
We will get you the complete list with complete details as soon as we have it ourselves.