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The Scions of Guilliman rule states that when a combat doctrine is enacted, all ultramarine models in the army are affected.

If my army consists of a Demi-Company formation and a 1st Company task force formation, and i enact the tactical doctrine that i gain from the Demi-Company rules, do the 1st company models gain the re-rolls too?

Normally, for any other chapter, the rules in the demi-company clearly state that only the models in the demi-company are affected.
But as per RAW, shouldn't all Ultramarine models get the re-rolls too, including the 1st company, since a combat doctrine is enacted regardless of its source?

With Ultramarines Chapter Tactics and a Battle Demi-Company (and with a 1st Company Task Force Formation you've also got a Gladius Strike Force) you've got several opportunities to use Combat Doctrines:

Once each for Devastator, Assault and Tactical Doctrines from 'Scions of Guilliman', which affect all Ultramarines models in the army.

Once for the Tactical Doctrine from the Battle Demi-Company's 'Tactical Flexibility', which affects all models with the Chapter Tactics rule that are part of a Battle Demi-Company.

Once each for Devastator, Assault and Tactical Doctrines from 'Codex Astartes', which affects all models with the Chapter Tactics rule that are part of a Gladius Strike Force.

They don't cross over to each other; each Combat Doctrine enacted affects only the affected unit as per how you chose or were able to enact it.

So if you enact the Tactical Doctrine from 'Tactical Flexibility' it will affect only models with the Chapter Tactics rule that are part of a Battle Demi-Company, per the rules for Tactical Flexibility. It's only if you enact a doctrine using 'Scions of Guilliman' that it will affect all Ultramarines units in the army, including those from a 1st Company Task Force if they have Ultramarines Chapter Tactics.

Agreed with the above, but as it is written, it seems they do cross over
The argument would be than even the tactical doctrine from the demi-company affects the whole army as per the scions of guilliman rule.

Is a combat doctrine enacted? Yes. Then all Ultramarine models are affected.
Another case of RAW vs RAI?

The beginning of the Combat Doctrines rule tells us:

"Several special rules allow units or Formations to enact one or more of the following Combat Doctrines. Whenever a rule allows this, it will note which units are affected and how many times per game the Doctrine(s) can be enacted."

When 'Scions of Guilliman states, "When one of these Combat Doctrines is enacted..." it's referring to a Devastator, Assault or Tactical Doctrine enacted by virtue of the Scions of Guilliman rule, and the affected units applies only to that rule.

It's not a blanket statement of enacted doctrines applying to all Ultramarines models. To claim it is would be cherry picking half of a rule and removing its context, which would be '(Selective Part of )Rules as Written'

It goes like this:

You can use the tactical doctrine 3 times in 1 game(assuming only 1 demi-company, more on this in a moment); depending on which source you use for the doctrine it will effect different units. So if you use the gladius or um source: all units in both the demi and 1st companies will be effected. If you use the demicompany source then only the demicompany units will be effected.

Now if you were to take 2 demicompanies you get 4 options to use each doctrine, but if you use 1 demicompany's tactical doctrine, it will only effect units from that demicompany not the other nor the 1st company units.

No matter how many doctrines you have available you can only use 1 per turn(the rules only give you permission to use 1, and specifically state that: "You can enact a single available combat dotrine at the start of each of your turns").

There is no other way to look at it. They do cross over. Read the ultra marines chapter tactics.

By my reading of it, there is an ambiguity in the Scions of Guilliman rule: is that second sentence only referring to "these combat doctrines" from the last sentence, or does the grammar of the sentence not necessarily connect it to the last?

IMO that proviso in the Combat Doctrines section seems to indicate that the RAI is to segregate the effects of combat doctrines when it's being invoked by the special rules of a sub-formation such as a demicompany. Otherwise, Ultramarines get a cascading benefit from sub-formations declaring combat doctrines - that's a powerful benefit, and something that I think the rule writers would be more explicit about if that was the intent.

 Xenomancers wrote:
There is no other way to look at it. They do cross over. Read the ultra marines chapter tactics.

We have; and we have, more impirrantly, read the combat doctrine rules.

Ultramarines combat doctrines are separate from demi-company dictrines, and those are both separate still from another demi-company's doctrines.

The base combat doctrines rule tells us this; the ultramarines ct gives you another set of them, and that set effects all ultramarines. The gladius and demicompany sets only effect the units therein.

Combat Doctrines called from Scions of Guilliman specifically states all Ultramarines in army. So this works on every UM unit in every your detachment.

Combat Doctrine called from Tactical Flexibility (Battle Demi-Company) or Codex Astartes (Gladius Strike Force) affects units only from this formations.

Ultramarines Chpater Tactics has nothing to do with Tactical Flexibility or Codex Astartes - those doctrines are "called" by Formations Command Benefits not by Scions of Guilliman.