Because video editing software (such as iMovie) obliterates overwrites many of the original tags (such as the shoot date of the video clip), I've been using -tagsFromFile to copy back the tags from the original source to the edited video.
What I have noticed is that after using -tagsFromFile, the target (edited video) file contains many tag groups of the form xmp-something_or_other, but these tags don't exist in the source!
This confuses me, as I thought that -tagsFromFile would not create any tags in the destination unless they existed SOMEWHERE in the source.
[FWIW, if I specify -all:all, then this doesn't happen. No additional tags not in the source are created in the destination]
In case this matters, I have checked and there are no .xmp files in the source directory.
Exiftool may not be able to copy the original tags to the original places, especially in the case of video. For example, if the tag is a Quicktime group tag, odds are that exiftool won't be able to copy it directly, as it can't write many Quicktime tags. It then tries to copy it a tag it can write, which is often XMP in the case of video.