I am taggin music by Genre separating by semicol the different voice so I an assign multiple cross-referenced genres like in https://www.dropbox.com/s/fpmzspomuz5gqub/Screenshot%202017-07-06%2014.43.19.png?dl=0
This is the genre field but I would like tagging also by instrumetal ytpe (e.g. orchestra;Piano or harpsichord;Violin etc.)
I was thinking to use the Gouping field (it is a spare field right) but it does not shows up like Genre, i,e, does not expand the single generes as shown in the picture
Any suggestion on how to efficiently implement my requirements?
You should learn a bit about defining and using views before you put a lot of effort into tagging. You populate tags to make it easier to browse and select music files. JRiver MC comes with some predefined views but you can create your own. From what you have said, I don't think that the standard album view will do what you want.
2/3 or more of my music library is classical music. I defined the view shown in the screenshot below for classical music written by major composers (a list that I defined.)
The view definition selects genre = classical and Composer in my list of major composers. This is a pane style view. The individual panes have lists of values for Sub_genre, Composer, Work Name, Artist and Version. My Sub_genre tag does something like what you wanted. I can narrow the list that is displayed by choosing a sub_genre value. Or a Composer Value. Or a Work Name or Artist or Version. I can narrow the selection down using any combination of those tags.
To define a view, right click on the word "Audio" and choose "Add library view". My screenshot was made several years ago so things look a bit different now.
I defined user fields (tags) for Sub_genre, Work Name, Version, Version and a few others. Then I created views to make use of those fields.