This option, found on the View Menu, allows you to view events in the Event List that have been fully acknowledged. Acknowledged events are shown in the event list with a check mark in the first column.
Fully acknowledged events include One Time events that have been acknowledged and recurring events in which all occurrences have been acknowledged. For recurring events, this can only happen if a Repeat Length has been set to a value other than Forever, since events set to recur forever will never be fully acknowledged.