Mortals can enter contracts with demons to gain strength, yet lose ownership of their souls, this is the only exception to the soul potential cap, because demons don't use mana, they use demonic energy, and once you lose ownership of your soul, its no longer tied to you, and the contract makes you not able to use mana anymore, but use demonic energy instead, meaning that, for example, lets say you have a tier-6 innate ability and a soul potential rank of A, you can enter a contract with an S rank or above demon, lose your soul and get pushed to A+ or even S rank, but it corrupts you, meaning you feel a desire for destruction and engaging in sins(depending on which demon you contracted with and which demon lords banner they're under.
Demonic contracts overwrite the soul with a sin-core, a conceptual tether to a specific demon's domain (e.g. Wrath for Satan and any demons under the banner of the demon lord of Wrath(each demon lord has demons under they're banner and are typically tied to the lords sin(i.e. incubus/succubi under Asmodeus)). This becomes the new anchor for energy progression. And also the sin-core limit is dependent on which demon you made a contract with, the higher rank the demon, the higher rank in potential the sin-core. Also because mana is directly linked to the soul, once you lose ownership via demonic contract, naturally you cant use mana anymore, and because you lose your soul ownership, the soul rank potential was never altered, you instead gave it up. Basically:
Soul isn't upgraded → it's forfeited.
Mana is lost → replaced by demonic energy, which is anchored to the demon's sin-concept via the sin-core.
Sin-core progression is based on the power of the demon you contract with — and once you're locked into that path, there's no return.
Sin-core Tiers are as follows:
Mortal Ranks:
Same as normal mortal ranks(G- to SSS+)
Only contracts with the one of the 72 keys of Solomon result in SSS+ rank potential
God Ranks:
Tier-1 all the way up to Tier-10
Only contracts with one of the 7 Demon lords results in being able to have the potential to cross the threshold into the Gods.