had developed and was selling a PC-based UNIX until 2000, when it then resold its UNIX assets to Caldera Systems, which later reorganized into Caldera International and changed its name to SCO Group. Through this chain of sales, SCO claims to be the "owner of UNIX". The validity of these claims is hotly contested by others. SCO claims copyright to all UNIX code developed by USL, referred to as SVRx, and licensing contracts originating with AT&T, saying that these are inherited through the same chain of sales. The primary document SCO presents as evidence of these claims is the "Asset Purchase Agreement", defining the sale between Novell and the Santa Cruz Operation. SCO says that this includes all copyrights to the UNIX code base and contractual rights to the licensing base. The other parties disagree. UNIX copyrights ownership The status of copyrights from USL is murky, since UNIX code is a compilation of elements with different copyright histories. Some code was released without copyright notice before the Copyright Act of 1976 made copyright automatic. This code may be in the public domain and not subject to copyright claims. Other code is affected by the USL v. BSDi case, and is covered by the BSD License. Groklaw uncovered an old settlement made between Unix System Laboratories (USL) and The University of California in the case of USL v. BSDi. This settlement ended a copyright infringement suit against the university for making BSD source code freely available that USL felt infringed their copyrights. The university filed a counter suit, saying that USL had taken BSD source code and put it in UNIX without properly acknowledging the university's copyright. This settlement muddies the question of SCO's ownership of major parts of the UNIX source code. This uncertainty is particularly significant in regard to SCO's claims against Linux, which uses some BSD code. Novell challenges SCO's interpretation of the purchase agreement. In response to a letter SCO sent to 1500 companies on May 12, 2003, Novell exchanged a series of letters with SCO beginning in May 2003, claiming that the copyrights for the core UNIX System V were not included in the asset purchase agreement and are retained by Novell. In October 2003, Novell registered those copyrights with the US Copyright Office. In response to these challenges from Novell, SCO filed a "slander of title" suit against Novell, SCO v. Novell. This claimed that