Grimoires and Vaults are catalogs of spells and items. They are collectively known as “Permissions”.
Normally one first buys access to a grimoire or a vault, and then buys spells or items from it.
An exception is that all Darks have a special unique grimoire that is tailored to their signature and aspect (ch.21). Until ch. 37, Mai's was thoroughly glitched. This was because of her “Unawakened Signature” (ch.32), and was fixed when she awakened her signature. Whether this is true for all new Darks is unknown. Note that just having this or any other grimoire or vault does nothing for the owner; you have to spend points to buy something that it is in it for it to have any effect.
A few MGs get a unique grimoire at level 100; those that don't get one at level 200. For Darks, this is in addition to their initial unique grimoire, giving them two. (ch. 36)
[The main bonus of a Unique Grimoire is the fact they synergize well with your abilities or offer unique side effects not commonly seen. For example, your Class Unique Grimoire will contain a multitude of spells directly correlating to the bonuses your chosen Class gives you. This means your spells will be much more effective compared to ones from other Grimoires. Beyond that, the spells within Unique Grimoires are usually more powerful or have special traits to them that give them an edge over normal spells. For example, when compared to a basic “Fire Magic” Grimoire, a unique one called “Grimoire of the Raging Phoenix” might offer fire spells that spread and burn more easily and with more explosive force. The appearance of said fire would also be different from a standard fire.] Selene, ch. 36.
List of vaults readers know is here. List of grimoires readers know is here.
A correction from Mika, 8/25/2024 6:27 pm, mgg-general:
I made an oopsie on Chapter 5. First, I forgot to explain that Vault/Grimoire rarities are locked behind major milestones (every hundredth level). So at level 1-99, Guardians only can buy tier one (aka common rarity) things. At 100 they unlock the next tier, 200 the one after that, and etc. Secondly, I also had more tiers/rarities shown than there should have been. Seeing as the highest level Guardians are only in the 500s, there should only be six known rarities. That has been fixed, so the current rarities are now Common, Rare, Masterwork, Arcane, Legendary, Mythic.
Here is what was originally posted:
[The Grimoire and Vault section shows what you have spent points to gain access to. Grimoires and Vaults can be viewed as “catalogs” for magical spells and physical items respectively. You must unlock them to first buy the items they contain, just as you did with the First Aid Vault (Common). The (Common) tag indicates its rarity as the first of eight tiers, each tier providing a selection of better items or spells but also costing more.]
[Common is first, then Rare, Masterwork, Arcane, Legendary, Mythic, Ascendant, and finally Divine. There is also a second set of tags used for more… esoteric items. The Arcane System, while useful, also has some oddities. Magic is not something that can be entirely codified or neatly labeled, and as such, the System sometimes does surprising things in its translations. The esoteric rarities are used to help label such idiosyncrasies.] Selene, ch.5
tutpuppy figured out what Mai's grimoire read before it was unlocked (comments, RR chapter 16): “Error: unable to access” in a strange font, overwritten by strange symbols in a stranger font.