Sony specifically forbids the use of online passes by developers on PlayStation 4 titles; as users must have a PlayStation Plus subscription to use online multiplayer on most PS4 games, Sony did not want publishers to add additional DRM designed to charge users further. Textbooks A similar system has been recently used by textbook publishers, by requiring the use of an online pass to offer supplemental content; including an online message board, an assignment system where homework may be assigned or handed in, or even an e-book version of the title. It is similarly intended to discourage the second-hand re-sale or sharing of these textbooks by only allowing their online functionality to be utilized by a single student; requiring others to buy their own copy of the book or obtain an access code individually. See also Damaged good Digital rights management References Digital rights management Online games