I find it interesting how Nasu removed the restriction Arcueid had in those old Q&A sessions. Her backup works by her scanning the area and measuring her opponent's strength,
including any weapon they might possess and then raising her level enough to
annihilate her opponent based on her own individual judgement. She can even use her back up strategically, in case she believes the opponent is hiding a trump card
she can raise her power even further. All this is part of her abilities as Ciel predicted, and her goal was to render her unconscious using an undetectable attack
before destroying her with all the power she could muster.
It's stated True Ancestors have no limits but curiously Arcueid couldn't do this thing
where she connected with the planet like this in her own route to repair her damaged vessel. And I think it had to do with her weakened state, since it's also mentioned
she couldn't use Marble Phantasm, which is analogous to synchronizing with the world, for this same reason. Seems
she needs at least 30% of Lifescale to use these abilities, which makes sense when you consider how many Authorities you can hold depend on Lifescale.
I also find it interesting how TAs are stated to have no limits to their power. With the only limitation being the speed and amount they can draw. From Katatsuki Kouhon:
As they have ceased to be free of self-interest—such as noncorporeal elementals ought to be—they have lost sight of their role as protectors, to the point that, among them, some have even become like humans, and gained independence from the natural world.
The True Ancestors have no limits.
As a result of being provided back-up from the planet, if the need should arise, they can strengthen their abilities until that need is met (however, the amount and speed that each True Ancestor can increase it by differs per individual).
But other incarnated elementals like say Meiren are explicitly stated to have a limit:
Incarnated Elemental: A
As an Elemental who shares mana with nature, she doesn't store magical energy inside her body, but rather she draws limitless energy from external sources. However, as she is bound by a humanoid form, there is a limit in how much output she can generate despite her infinite supply, and the amount of magical energy that she can make use is limited within the range of a humanoid Heroic Spirit.
Which I guess makes sense since she is only stated to be on a category parallel to True Ancestors. Kinda like dolphins and whales.
In contrast True Ancestors have the ability to transform their vessel at will. I imagine it's kinda like having a hose and being able to change the size of the hose as well as the speed. Though the likes of Yu Meiren don't seem to have that capability, could be that's the limit of her Servant form though.
