If you define logoffuri in your ACO, when that agent finds the request received matches the logoffuri then it will logout the user by removing the user session and the SMSESSION cookie gets set to LOGGEDOFF.
If you did not define the logoffuri and want to termiate user session, you can let the user access an fcc file which contains the following.
@smlogout=true
@target=/whicheverpageyouwant.html
This forces the agent to logout the user session and the SMSESSION cookie gets set to LOGGEDOFF.
In the above use case, it will also redirect the user to the defined target after the logout.
It is just much easier to use logoffuri.