CoS and DSCP are both methods of marking packets for quality of service (QoS) purposes. QoS is a mechanism that allows network devices to prioritize and differentiate traffic based on certain criteria, such as application type, source, destination, etc. CoS stands for Class of Service and is a 3-bit field in the 802.1Q VLAN tag header. CoS can only be used on Ethernet frames that have a VLAN tag, and it can only be preserved within a single VLAN domain. DSCP stands for Differentiated Services Code Point and is a 6-bit field in the IP header. DSCP can be used on any IP packet, regardless of the underlying layer 2 technology, and it can be preserved throughout the IP packet flow, unless it is modified by intermediate devices.
References:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos/configuration/15-mt/qos-15-mt-book/qos- overview.html
https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html
https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-packet-marking/10103- dscpvalues.html