[ Thanks to Dmitri Popov for this link. ]
“In addition to writing tags, ratings, labels, descriptions, etc. directly into the photos, version 2.0 of digiKam can save metadata in a separate .xmp file. This approach has several advantages. First off, it speeds things up, as writing data to a text file is faster than embedding metadata into photos. This also allows you to store metadata for RAW files, since writing metadata directly to RAW files can sometimes be problematic.”