The Association of American Publishers and The Authors Guild Inc. have sued Google Inc. for copyright infringement related to its Book Search program. Book Search involves digitizing a planned 2.5 million books from the University of California, in addition to many other books, including non-English books. Books that are acquired from their publishers are treated differently by Google than those acquired from its library partners. Books from publishers can carry some advertising content, which offers a stream of revenue to publishers, and they include links to the publishers' websites. Google has been moving toward user-paid access for some time, and it has been building up a database of registered users through services like Gmail. Questions have arisen about the legal status of books in Google Book Search concerning their 'in-print' or 'out of print' status, since authors control copyright when a book is out of print. Some attorneys believe that inclusion in Book Search constitutes in-print status, and with a print-on-demand option, they could remain in print forever, contractually. Copyright holders could license content and collect money on books they did not digitize themselves, but questions remain as to who those copyright holders would be. Reversion clauses differ from one contract to another, so there are no absolute standards to define what 'out of print' means.