Downloaded Music

When I go to my apple music settings on my iPhone 7, there is an option called "optimize storage" that says "if your iPhone is low on space, automatically remove music from this iPhone that you haven't played in a while.

I was just wondering if this means the music will be completely removed from my library or if it just means it will undownload the music so it won't take up as much storage.

If it means completely removed from my library, is there an option to automatically undownload music I haven't played in a long time, while still keeping it in my library? Thank you for your help!