Addendum refers to extra pages added to a book. Along with the Errata, which are corrections to mistakes in a book, they are additions to a book that are given out after its initial release. Addendum is a fitting word: it adds more pages to your Grimoire, allowing you more spells to cast, and it was released after Scholar's initial release.
/ja "Addendum: Black" <me>
The addendum grants access to the following spells: