1) Class of service The indication of how an upper-layer protocol requires that a lower-layer protocol treat its messages. In SNA subarea routing, COS definitions are used by subarea nodes to determine the optimal route to establish a given session. A COS definition comprises a virtual route number and a transmission priority field. Also called TOS (type of service). 2) Corporation for Open Systems An organization that promulgates the use of OSI protocols through conformance testing, certification, and related activities.