CoA stands for Change of Authorization, which is a feature that allows a RADIUS server to adjust an active client session after it is authenticated. For example, CoA can be used to reauthenticate a client, terminate a client session, or change the VLAN or group policy of a client. CoA is supported by several RADIUS vendors, including Cisco ISE. CoA is defined in RFC 5176 and uses a pushed model, where the request originates from the RADIUS server and is sent to the network device that acts as a listener. CoA requests can have two possible response codes: CoA-ACK (acknowledgment) or CoA-NAK (non-acknowledgment).Reference:=

Some possible references are:

RADIUS Change of Authorization

Change of Authorization with RADIUS (CoA) on MR Access Points

Change of Authorization with RADIUS (CoA) on MS Switches

Technical Tip: Radius COA behavior