That information is something MakeMKV "guesses" by trying to run the BD-J stuff on the disc and extracting information from it. Believe me or don't, thats where it comes from. If you put enough effort into trying to interogate the BD-J menus, you can come up with some pretty good guesses. But its not quite simply some text field in some file you can read.
Try removing your Java, and that information magically disappears.
I have written Blu-ray Menu support for Media Center, I know what information is on the disc, and what is not.
That information is something MakeMKV "guesses" by trying to run the BD-J stuff on the disc and extracting information from it. Believe me or don't, thats where it comes from.
I 100% trust you now that you provide an explanation that makes sense
I don't care how makemkv gets this information from, but it is clearly, as I said, from the disc. It doesn't simply make it up based on length, etc as you initially stated, because there are other tracks which are the same length, that are discarded while this track with this comment is retained, and I don't think it's by chance. Also the comments on the other tracks are accurate too.
The only thing I can tell you is that this FPV_MainFeature comment is present on quite a few titles, and accessing this comment is clearly useful in selecting the main title when there are many tracks with the same length and the same number of chapters.
In the case of The Spy Who Dumped Me, JRiver (and makemkv) selects the correct playlist anyway, but 3lled00d's tool (and MPC-BE) don't. So I was mainly providing the info to 3ll3d00d so that he can try to improve his libbluray algo and select the track that JRiver selects more often.
Ghostbuster 2016 provides an interesting example.
In that case, the main playlist identified by libbluray (in 3ll3d00d's tool) and MPC-BE is 00001.mpls, and based on the information (playlist name, Immersive HD Audio present, number of chapters) it's likely to be the correct one.
However, JRiver selects another track, and until you add the ability to display the .mpls track name I won't be able to find which one because there are too many options. Here are the tracks discarded by makemkv because they have the same length as 00001.mpls:
File 00001.mpls was added as title #0
File 00102.mpls was added as title #1
Title 00506.mpls(1) is equal to title 00001.mpls and was skipped
Title 00506.mpls(2) is equal to title 00001.mpls and was skipped
Title 00507.mpls(4) is equal to title 00001.mpls and was skipped
Title 00508.mpls(4) is equal to title 00001.mpls and was skipped
Title 00509.mpls(4) is equal to title 00001.mpls and was skipped
Title 00510.mpls(1) is equal to title 00001.mpls and was skipped
Title 00510.mpls(2) is equal to title 00001.mpls and was skipped
Title 00511.mpls(4) is equal to title 00001.mpls and was skipped
Title 00512.mpls(4) is equal to title 00001.mpls and was skipped
Title 00513.mpls(4) is equal to title 00001.mpls and was skipped
Title 00514.mpls is equal to title 00001.mpls and was skipped
Title 00515.mpls is equal to title 00001.mpls and was skipped
Title 00001.m2ts is equal to title 00001.mpls and was skipped
In that case, provided that 00001.mpls is correct as I suspect it is, then JRiver might have guessed it wrong.