Premium shares from Rez!

Premium e-books are epub 3 e-books for our official releases, and which have higher quality images than the ones you can download on Kindle and other stores.

They are not available for normal purchase, and can only be downloaded by purchasing them using the credits that Premium Members get once a month.

Premium E-books may also have extra bonus content, like special illustrations or translator notes sections, extra short stories, etc, depending on the specific title and availability. We’re going to work hard to make these really special, and we’ll announce these extra features in our newsletter, twitter, and forums before publication.

From: https://j-novel.club/faq