Activity Stream enables agents to communicate with requesters and make
internal notes about the work done on a record. Internal notes are only visible to
fellow agents. External comments are visible to agents and requesters.
In Activity Stream, agents can embed knowledge articles, import
solutions from Agent Assist, and ask requesters for more information.

You can set system properties that give agents the option of using a:
Once you use system properties to enable these options, the sliders in the
configuration menu appear.