Your genres and Roon’s can be mixed together in any way you like. Roon’s genres have some structure out of the box (meaning Bop is filled under Jazz, Concerto is under Classical, and so on). The idea here is to show something that’s comprehensive and comprehensible when you first use Roon, and then let you go in whatever direction you want.
In Settings you can turn on both your tag genres and Roon’s genres, and this is set per install – if others in your household just want the basics, and you want nothing to do with them, you can set this on your individual devices.
All genres have editable “parent genres” meaning if you consider Concerto to be a top level genre (as opposed to being filed under Classical) you can edit it that way. Similarly, if your tag genres don’t exist in Roon’s hierarchy, you can leave them top level, or file them as subgenres however you please.
This is really a matter of preference. For me personally, I use the genre browser when I don’t know what I want to listen to, and being able to drill down from a finite number of top level genres makes things easier for me. It also makes the whole screen usable for my wife, who likes to explore whats in the collection, and may know she wants to play some jazz even if she doesn’t necessarily know every subgenre.
I think in most cases, you’ll want to go to the genre, click the little pencil, and set a parent genre – this would allow you see Classical at the top level, but with your 20 subgenres underneath it.
Mapping is a little counter-intuitive in that it’s really for genres you don’t want shown in your library. For example, in my file tags, I noticed more than 10 variations on R&B, none of which were particularly useful (General R&B, really?). I mapped these all to R&B, and now even if those albums aren’t identified by Roon, they all show up properly in my collection, and I can shuffle them all by going to the R&B page and hitting play.
Let me know if that answers your question @hshrader!