We can distinguish UNIX the trademark from Unix the code-base.
Unix was initially developed at Bell Labs, owned by AT&T. This Unix team became AT&T's Unix System Laboratories (USL) and produced Unix System V (Roman numeral for five) or SysV for short. The University of California at Berkeley (UCB) also licenced Unix for academic use, their Computer Systems Research Group (CSRG) later made many important changes and additions (notably TCP/IP) in their Berkeley Software Distribution (BSD) which were later incorporated into many descendants of Unix leading to the BSD vs SysV split. Ultimately a lot of the BSD changes were back-ported into SysV (which we can consider the main "ancestral Unix" code base).
Along the way, many different businesses have licenced this code-base (at various stages in it's development) and used it as the basis of their proprietary Unix operating systems - AIX, HPUX, IRIX, Solaris, Ultrix and dozens of others.
USL was purchased by Novell. At this time, the ancestral Unix code was known as Unix system V release 4 - or SVR4 for short. Novell named their product Unixware to complement the name of their legacy network OS Netware.
The Santa Cruz Organisation
Novell eventually sold their Unix business to an old SVR3.2 licensee The Santa Cruz Organization (SCO) whose main business up to that point was selling a product named OpenServer that was based on Unix SVR3.2
The Santa Cruz Organisation later sold their Unix business to a Linux company Caldera who later renamed themselves to The SCO Group (sometimes referred to as new SCO or SCOG) and who had a disastrous failure of leadership leading to chapter-11 bankruptcy and sale of the Unix business to UnXis, a business formed for this purpose. Subsequently The SCO Group were reorganised into TSG Group Inc and TSG Operations Inc. They have no role regarding maintenance of the ancestral Unix code base.
So now UnXis are responsible for marketing and developing/maintaining Unixware - the ancestral AT&T Unix code base. Because the Santa Cruz Organisation (old SCO) originally ported† Unix to the x86 platform, I believe x86 and x86_64 are the only target platforms that UnXis directly support. Novell (since bought by Attachmate) still own some rights‡ to Unix but do not do any work on the source code.
† Microsoft licensed Unix and ported it to 16-bit Zilog Z8000 - old SCO purchased Xenix from them and ported it to the 16-bit 8086 architecture (used by IBM for their original IBM PC). Old SCO later ported SVR3.2 to x86 as 32-bit SCO-Unix later renamed OpenServer
‡ Novell's rights were contested, somewhat futilely, by The SCO Group (now named TSG Group Inc), the bankrupt remnants of the old Linux company Caldera. It is not yet clear whether TSG Group Inc have finally discontinued this and related litigation as a result of August 30, 2011 court decisions against them