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Can someone help me understand how the new Marine Codex works in relation to the Black Templar Datasheet download?

All units, including Black Templar units, have "Oath of Moment." Would this imply I can use the Codex Marine "Oath of Moment" Detachment Rule, in addition to the "Templar Vows?"

Part of it doesn't feel right and I'm a bit confused.

Oath of Moment is your Faction Ability.

Templar Vows is the detachment ability of the Righteous Crusaders detachment. You get it instead of any other detachment abilities.

You pick a faction (in this case Space Marines) and you get their faction ability (Oath of moment).

You then Pick a Detachment, space marines can choose any of those in the codex of from the various chapter index ones - you get the additional rules, enhancements and stratagems specific to that detachment.

Some Space Marine units have a Chapter Keyword (Black Tempar for the units in the BT index) you cannot include units with more than one chapter keyword in your army.

And some of the Detachments restrict which units you can take.