I think, but I am not sure, that only fields of type "list" can be nested. Just enter the tag information like this: AAA\BBB\CCC\DDD\EEE. You can nest as many levels you want, so you could have:
Voice\Soprano\Maria Callas\Role1
Voice\Soprano\Maria Callas\Role2
Voice\Soprano\Maria Callas\Role3
If you customized your view with panes you will get a tree list of the nested tags. So if you select Role3 you will get all the operas where Maria Callas performed Role3. As I have only a few opera in my libraries, I do not specifiy the role of a singer in an opera.
Take the field "soloists" as it is a list field. The field "instrument" is a string field. You can only add one instrument, which does not make much sense. I proposed once to make this field also a list field, but it never happened. But if you get it how to nest fields, then you can create your own schemes that work for you. You can also create your own list type fields.