Even though I use the File Output Node frequently, there's an option I've been ignoring but finally have to ask about:
Use Node Format
The File Output Node seems to work just fine without this box ticked, so what does it do?
Apparently I'm not the only one. There is also this decade old Blender Artists post where a user observes: “I noticed an option to use node format… it was checked by default, so I just left it. But if anyone knows, why would you not have that checked?” Nobody replied.
If I had to guess, I would suspect that the Base Path settings are global and apply to all inputs, whereas the Subpath settings are individualized for each local input - so this is only a consideration if you have more than one input and have enabled the Use Node Format option. But this is just my intuition, and I don't know much about the workflows of others using the File Output Node. The Blender Manual's page on the File Output Node doesn't presently go into detail about the two different sets of settings.
Assuming my above guess is accurate, it would be less confusing if enabling Use Node Format caused the Base Path settings to grey out. And if when disabled, the File Subpath settings would grey out.
I'm in the habit of using one File Output Node per image anyway (as opposed to a single one with multiple inputs). And that's probably why I've gone so long without thinking deeply about the two sets of settings. Now I have to wonder if there is any advantage to using a single File Output Node (memory conservation?). But I suppose that would be a separate question.
Screenshot of the File Output Node's settings in the N Panel (and scene Output settings) for reference: