I have a document library and I have created two columns, one area and the other subarea. When a document is added, I want the person to select the area and then have the subarea list depend on the area selection. Is this possible?
This isn't supported out of the box.
You have a few options:
Use an InfoPath form to apply the filtering, but keep in mind that InfoPath is being discontinued in future versions
Perhaps adapt the requirements a bit and use a Managed Metadata column where the person selects the sub-area (which is configured as a child node of the area - effectively selecting two metadatas in one)