What causes the "file" property to appear?

does anyone know the purpose of the file property?
all hls pages have file-path:, but only a few of them have file:. the last time I saw it I just deleted that property to see if it breaks something… nope, to this day nothing’s broken as far as I know (even across reindexes).

is it some deprecated remnant like title:? if not, what does it do, and what determines that some pages should have it and others don’t?