Because UNIX systems communicate through industry-standard TCP/IP communication protocols, and use an industry-standard interface based on an X-Windows and Motif graphical environment, and share disk drives through the use of industry-standard NFS technology, they are called "open systems".
Advanced technology is shared between UNIX vendors to create more flexible solutions for clients. UNIX operating systems typically start with source code that is now licensed by Novell, Inc. Novell's UNIX Systems Group licenses the actual UNIX source code to vendors such as IBM, Hewlett-Packard, Sun Microsystems, SCO (Santa Cruz Operation), and many others. (The UNIX source code was owned by AT&T and UNIX Systems Laboratories until 1994.)
Also, the API's (Application Programming Interface) for these operating systems are published by the software vendors, and are very consistent. It is generally a simple task to port a software application from one vendor's UNIX platform to another. This creates a lot of competition in the UNIX marketplace, which is good for computing customers.
This sharing of technology between UNIX vendors is in stark contrast to other vendors operating systems, which are labeled "proprietary".