Forced subtitles
A forced subtitle is a term used in Open Caption subtitling. It refers to a subtitle that should be displayed on the viewer's TV screen even if they have subtitles turned off.
Forced flags are used when exporting a file with the DVD Builder (where a DVD export format supports the use of forced subtitles).
Forced flags are also used in Q-Live to allow Dual Open Caption output.
You can make the current subtitle forced by selecting Edit properties... Force On from the right-click menu. You can also use the "toggle force" button 
located on the file toolbar. A forced subtitle is indicated by an F in the subtitle display as shown below.
To remove the force flag from the current subtitle, repeat the above operation to toggle the flag off.
To remove all force flags from the current file, select Edit properties... Clear all Forced from the right-click menu.
To remove forced flags from a range of subtitles, first switch to the Compact View and select a range of subtitles.