The objective of such differentiation is generally associated with assigning priorities to the data payload or access levels to the telephone call.
The field specifies a priority value between 0 and 7, more commonly known as CS0 through CS7, that can be used by quality of service (QoS) disciplines to differentiate and shape/police network traffic.
Most Internet Service Providers do not trust incoming QoS markings from their customers, so COS is generally limited to use within an organization's intranet.
As related to legacy telephone systems, COS is often used to define the permissions an extension will have on a PBX or Centrex.
Cisco implements such classification through either access-lists or Network-Based Application Recognition (NBAR).