A brand kit is a look you keep: the cover's layout (type sizes, spacing, where the title and author sit, the text's colour and shadow, the back and spine's colour, where the emblem goes), the interior's type style, the ebook's look, the scene breaks and the emblem.
Make one from a book that looks right. On the book's page, under Brand kit, name the kit and choose Save as a kit. Name a series, or keep it for the book's pen name, and it's offered first on their books.
Give it to other books. On another book's page, choose the kit and Give it to this book. Each format takes what it can (a vertical Japanese edition keeps its own cover layout, made for vertical lines), each edition keeps its own words on the cover, and the cover's layout is fitted to the book's trim. The cover editor opens with the new layout; the files built before are out of date, and Rebuild books builds them again in one run.
Change a kit. On a book you've changed, choose Take this book's look, then give it again to every book that took it, in one go. Removing a kit leaves its books as they are.
The cover's typefaces follow the edition's script for now; a kit's type is the interior's style and the ebook's look. Brand kits come with the Publisher plan.