The full list of attendance codes are available in: Register>Manage Register, click on View Register. At the top left of the screen you will see this icon:
Click on this and you are directed to an article containing all the Registration Codes, with their description (click here for the article).
You can also access this list on any student profile. On their attendance tab you will find the attendance reports on the right side, and the Attendance Codes are accessible from here too.