Here is the list of faction keywords that gain the Objective Secured special rule! Please note, this article pre-dates Chapter Approved and since that book’s publication, is no longer valid. Reference Chapter Approved for the current rules.
For convenience, here is the pertinent verbiage:
On August 9, 2017, the Warhammer Community site introduced the Objective Secured rule. Since Chapter Approved is still forthcoming, Games Workshop clarified for us which Factions gain Objective Secured for the ITC community.
If your army is Battle-forged, all Troops units in <Faction> Detachments gain the Objective Secured rule, so long as every unit in a detachment has the same faction keyword from the following list: Blood Angels, Dark Angels, Space Wolves, Deathwatch, Astra Militarum, Adeptus Mechanicus, Adeptus Ministorum, Asuryani, Drukhari, Harlequins, Chaos Daemons, Renegades & Heretics, Thousand Sons, Death Guard, Necrons, T’au Empire, Orks, <Hive Fleet>, and Genestealer Cults.
Troops units with Objective Secured that are within range of an objective marker (as specified in each mission’s rules) control it even if there are more enemy models within range of it. If an enemy unit within range of an objective marker has a similar ability, then it is controlled by the player who has the most models within range as normal.
Note: Codex: Grey Knights, Codex: Chaos Space Marines, and Codex: Space Marines are explicitly not covered by Objective Secured because their Codexes provide a similar rule (i.e. Defenders of Humanity).
These rules are effective immediately in the ITC. Also, you will see these in effect at many events around the world including the NOVA Open, SoCal Open, etc.
And to clarify this further, a detachment that was comprised entirely of say, Dark Angels, would get ObSec. A detachment with Dark Angels and an Assassin in it, would not. Every unit in the detachment has to have the same faction keyword from the list above to trigger the rule. If even one unit does not have the same faction keyword from the list above as every other unit in the same detachment, that detachment will not have ObSec.
Another example: a detachment wherein every unit has the <Black Legion> faction keyword would get ObSec but a detachment with <Black Legion> units and Brimstone Horrors would not. Note: if you summon in a unit, it is not a part of the detachment and therefore does not alter it’s composition. Summoned units would therefore not gain ObSec nor would they take it away from a detachment.
A third example: a detachment wherein every unit has the Asuryani faction keyword would gain ObSec, but one with Asuryani and Ynnari would not.