What are forced subtitles on Blu-ray disc?
Unlike DVD, on Blu-ray, each subtitle item has an attribute that specified whether particular subtitle appearance should be forced. Forced subtitles are shown regardless of current player setting, typically when movie contains a foreign language scene. The "forced" attribute is assigned per individual subtitle image, so entire subtitle track must be scanned to determine if forced subtitles are present.
MakeMKV creates two items for each blu-ray subtitle track in selection screen - regular track and "forced only" as indicated on this example.
In produced MKV file regular subtitle track will contain all subtitles and forced track will contain only these with forced flag. If entire original track is forced then both tracks will be identical. If no forced subtitles were in original track, the resulting "forced" track will be empty and will not be present in MKV at all.