Paths are typed to the node they fit

Not every field means something for every kind of node. A radio station has no album or year; a podcast episode has a subscription, a folder and a publish date but no album artist. So each path belongs to one of three categories — Standard (your music, filters, audiobooks, inbox and playlists, which all carry full tags), Radio, or Podcast — and that category decides two things at once: which fields you may group by, and which nodes the path can be applied to.

On the Paths tab the template list is drawn in two bands: Standard — where your own templates live, and where every new template is created — and a Reserved band holding the Radio and Podcasts templates. While you edit a Standard template the field menu offers everything; a Radio template offers only the fields a station actually has; a Podcast template only what an episode has — its folder, its subscription, and its publish year. On the View tab, applying a template only lights up the nodes it fits — the rest grey out and can't be ticked — so a Standard path can never land on the Radio or Podcasts nodes.

In fact you never apply anything to Radio or Podcasts at all: each is permanently paired with its category's template. Reshape the Radio or Podcasts template on the Paths tab and the node follows by itself; with a Reserved template selected, the Apply button is simply disabled, because there is nothing to apply. This stops you building a path that quietly produces nothing by grouping on a field the node can't fill — and since a followed template can never be deleted (see Discovery paths above), the Radio and Podcasts templates are always there to reshape.