Q: What is the difference between a keyword and a Faction keyword?
A: The only real difference is that Faction keywords are used when building an army; when Battle-forging an army, for instance, you will often only be able to include units in the same detachment if they share the same Faction keyword. Also, if you are playing a matched play game, you will need to have an Army Faction – this is a Faction keyword that is shared by all of the units
in your entire army (with the exception of those that are Unaligned). Once the battle has begun, there is no functional difference between a keyword and a Faction keyword.

For example, when creating a Battle-forged army for matched play, I take two Patrol Detachments; the first contains only units with the Heretic Astartes Faction keyword, and the second contains only units with the Daemon Faction keyword. My Army Faction is ‘Chaos’ because this is a Faction keyword every unit in the entire army shares.

Once the battle has begun, the distinction between keywords and Faction keywords no longer has any effect – both are used to interact with abilities identically. Imagine, then, that the Heretic Astartes Detachment contains a unit of Possessed (which does not have the Daemon Faction keyword, but does have the Daemon keyword), and I choose for them to replace their <Mark of Chaos> keyword with Khorne. If the Daemon Detachment contained a Herald of Khorne, his ability to ‘add 1 to the Strength characteristic of all Khorne Daemons’ would also apply to the unit of Possessed, as they have both the Khorne and Daemon keywords.