CD "Chapter" Solution?

In some CDs, most often in classical music, we find a "chapter", which characterizes the "theme", for a bunch of tracks.
For example, see here:

As you can see, there are 2 "chapters", devoted for the 2 CDs in this box.

Act One: "When Cats Are Maddened By The Midnight Dance"
Act Two: "Why Will The Summer Day Delay - When Will Time Flow Away"

Now, suppose, that I do not want to have 2 dedicated folders for those 2 CDs, where I enter this information.
I would rather, include this information in the MP3 metadata.
How would you solve this issue?

The CD does not have this trouble as you do not see any title information in the tracks at all, you have to read the inlet.
If you want to have this grouped, you may add the chapter name to the title - an action "Quick" of the type "Format value" for TITLE with the format string "When Cats Are Maddened By The Midnight Dance - %title%" adds that rather lengthy string in almost no time. But the titles are very hard to read then as you have to scroll to the end to see where the title is different from the others.
You could use the SUBTITLE for the chapter - the problem is that only very few players make use of this field. Also the comment is of little help.
Or you create your own ALBUM:
CATS 1 - When Cats Are Maddened By The Midnight Dance
CATS 2 - Why Will The Summer Day Delay - When Will Time Flow Away
What I would in any circumstances: use tracknumbers with values above 100 for the tracks on disc 1 and above 200 for disc 2...

So: there is no standard for this, the chapters are owed to the transport media.

Add a field/tag called "chapter" would be the best bet.

It's in the manual,

Standard Tags

Extended Tags

OK. Have to think...
If you look at MP3tag's default Extended Tags, would you pick one of those,
which in your opinion would be best suitable for this "chapter"?

Yes. Most probably that's my preferred solution...