I was about to back up my iMac but I was going through everything and deleting files I didn't need to back up. I went to System Settings to see how much space I had left, and saw that 40.7 GB is taken up by "Documents". This isn't an issue because I still have plenty of space, but my Documents directory is only 448 MB. So I was wondering what on Earth defines "Documents" in the storage options in System Settings?