Hello
I did not add this tag. It was added more than a decade ago in the phase when all Chilean roads were being imported from a government database. It means nothing in OSM and you can delete it. It refers to the surface, but the surface tag was added a bit after the import, so it came kind of redundant.
The roads that were unknown by the community that were not paved, remained with the tag unpaved. Maybe dirt or ground is the proper value, but we don´t know yet.
regards