The Last Action By filter group consists of a checkbox list with search filter:

With this filter, you can limit the displayed BI data to the nodes which have last audited actions made by a specific user or users.

Start typing a user account name you are looking for in the search control, and the list will be limited to user account names that contain your input.

Tip
The last audited action user is displayed for all BI objects in the Action By column of the Content Area, in a tooltip, and in the Audit History dialog window.

Warning
The information in the Action By column may differ from information in the Audit History window, because Audit History immediately displays the information in real-time which can be newer than information in MMDB.

 

There is an additional item (All) in the list of users. Use it to toggle between all user accounts being selected and all user accounts being unselected.

If all users are selected, or none of them, then the Last Action By filter is not applied, and it does not limit the data displayed.