"ABJ" stands for "Authorized Books of Japan".
Citing from Association for E-Publishing Business Solution (Japanese),
The ABJ mark is a trademark indicating that this e-bookstore and e-book distribution service is an authorized distribution service that has been licensed for use by the copyright holder. The aim was to provide an environment where readers can read and subscribe with peace of mind, and to promote the development of a healthy content market.
It means that the digital service provider (e.g. websites, apps) is officially authorized and has been given permission to use licensed works. Services that have this mark always mean that they are legal providers. However, services that do not have this mark does not mean that they are illegal (e.g. Crunchyroll Manga) since there is a procedure needs to be done to get the mark.
Some supplementing quotes in English regarding the mark:
Manga Planet - Updates on System Improvement:
- Addition of the ABJ mark and short description to the footer
The ABJ mark is a trademark indicating that this e-bookstore and e-book distribution service is an authorized distribution service that has been licensed for use by the copyright holder.
We will continue to release e-books with rights properly processed to contribute to the development of the manga market.
Beaglee Inc. - Non-Consolidated Financial Results
for the Fiscal Year Ended December 31, 2018 (PDF):
Moreover, the Company has obtained permission to use the ABJ mark that signifies it provides full version services with permission for use of contents granted by the copyright holders.
The whitelisted services are listed on this PDF file (updated irregularly). From left-to-right:
- Registration number
- Service name
- Operator (business company/person) - with the last updated date
Example of English manga sites that have been registered:
- Has ABJ mark: always legal
- Does not have ABJ mark: maybe legal, but most possibly illegal