The website contains a default list of item bonuses. But since your server could have new bonuses or different values for item bonuses, you can check and update the bonuses table.
For instance, if some bonus are missing you will see ??? in the item shop instead of the bonus name.
At the moment it is not possible to update this table from the admin panel, so you should do it from Navicat or a similar software.
To do so open the website database and look for the table called item_bonuses.
Inside this table you will find the list of all the bonuses supported by the website. You can add, remove or update the bonuses in the table. The bonus name will be in English and then you can translate it from the translation files.