MKV subtitles are either not marked ... or marked forced and/or default ... so when I refer to default and/or force subtitle, I refer to what is marked in MKV (not what is intended in Bluray/DVD). You can check which subtitles are marked with tools like MediaInfo
JRiver 23.0.22 was used to test this
If one of track is marked default ... JRiver will start play movie with that subtitle
If none of subtitles are marked default ... JRiver will start play movie with no subtitle
If one of subtitles is marked forced ... turning off subtitles will still play forced subtitle (in case where there is no subtitle marked default, JRiver will still start play movie with force subtitle). So in JRiver there is no way to turn off forced subtitles (some players like MPC-HC can turn off forced subtitles). This makes sense as watching movies like Avatar or Arrival needs force subtitles to understand alien language (so you don't want to accidently ruin your movie experience by switching force subtitle off).
So how subtitles behave in JRiver depends on how you ripped your source. You can retag subtitles with tools like MKVToolNix.
My usual approach to this is:
(1) if source has force subtitle ... I marked it as force and default in MKV ... also I put "Name" tag as "English(FS)" (so movie starts playin with forced subtitles in JRiver ... and it cannot be turn off ... if you want to be able to turn it off, don't mark it force)
(2) if source has no force subtitles ... I don't put any force or default tags (so movie starts playin without subtitles in JRiver)
(3) in some cases there are 2 non-force English subtitles ... normal one and SDH (for hearing impaired ... basically it subs background noises like [sigh], [loud steps]) ... I put "name" tag for SDH subtitles as "English(SDH)"
Reason I name tracks ... so as not to be confused when presented with 3 English subtitles options (having them displayed as "English(FS)", "English", "English (SDH)" simplifies things) ... If there is only one English subtitle and its SDH, I name it nothing (you don't have any options anyway).
Trick is in creating those. I will describe what I do with MakeMKV (refer to attached pictures). You have to understand MakeMKV behavior. For each subtitle, MakeMKV will try to determine if there are force ones embeded in them (this will be determined once you start creating MKV). Hence, you see option marked as "(forced only)" under each subtitle . If there are none force ones embeded and you have ticked that option, it just get ignored. If there are embeded forced ones and you have ticked that option, they will be extracted into separate subtitle track.
I never saw forced subtitles embeded into normal subtitle track. What I have seen so far is a separate track for just forced subtitles (as you can see in attached picture). But looking at MakeMKV screen, you have no idea whats going on. Watching movie to see what sutitles are is pain (unless you have perfect memory ... like in Arrival ... force subtitles where only present while talking to alien in the end). So I use Subtitle Edit tool to see what each subtitle track contains.
In attached picture example, first track was forced only (i ticked "(forced only)", but could have just ticked its parent ... ticking them both would have created 2 exact subtitle track). Note that I tag it ... name "English (FS)" and set default and forced flags ("MKVflags" value set as "df" ... if you put just "d" it will be just marked defalt ... if you put just "f" it will be just marked force).
In some cases forced subtitle track is below normal one (order wise) ... I like my force subtitle on top of the list ... So in those cases, in "Properties" i set "Order Weight" value to -50 to move it to top of the list (this is just personal preference).
Also note, for second subtitle track "(forced only)" was ticked (they all ticked by default). As I knew that track contained no embeded force ones, I knew it will return nothing and "(forced only)" would be ignored (was lazy to untick it).
How you browse movies and select subtitles is matter of personal choice ... if you just starting to convert your dvd and bluray collection for HTPC experience, determine early what you want and how you want it, to avoid re-ripping/re-muxing later
Hope this helps someone (spend 2 days to figure this out ... getting too old for this)