What does "Add to Library" and "Remove from Library" mean in Podcasts?

They're just podcasts. Episodes should be either downloaded or not downloaded, and podcasts should be either subscribed to not subscribed. But it seems an episode can be "in your library" without being downloaded, and a podcast can be "in your library" without being subscribed. What does that even mean? (I'm referring to Podcasts on both Mac OS and iOS.)