What is an "Art Song" in roon?

When I do a search on “art song” in roon I get a very eclectic mix from pop to jazz to classical crossing almost all conceivable genres.

But I also have a classical genre defined “art song” with various sub-genres like lieder, mélodie and american art song.

How does roon define “art song” for the non-genre based search? What will roon use in radio, for example, (genre or non-genre) when trying to build associations in the queue?