Part one of this article, which ran in the August issue of Engineered Systems, covered several issues, including how difficult it is to come up with a working definition of a proprietary bas. Many manufacturers and engineers don't agree on what exactly constitutes a proprietary system.
Obviously a proprietary bas is one that utilizes a company's proprietary protocol, but unlike older proprietary systems, today's systems can typically be linked, through gateways, routers, or portals, to other manufacturers' systems that may utilize BACnet, LonTalk, or other communication protocols.