400
This is the reason a student would see the message "Your user account does not yet have access to this course. To enroll in this course, you must log back in with a user account that does have access, or register this user account with an access code for this course" when trying to enroll in their course.
What is: The student’s access code is good for one book/edition but the course uses a different book/edition.
(This can be custom to standard, standard to custom, or change in edition or text.)