My reasoning for Exif was that it is not only auxiliary but also post-hoc. Exif was defined independently from image formats and only got adopted later because

My reasoning for Exif was that it is not only auxiliary but also post-hoc. Exif was defined independently from image formats and only got adopted later because those formats provided extension points (JPEG APP# markers, PNG chunks). You've got a good point that there are multiple types of metadata and some metadata might be crucial for interpreting data. I would say such "structural" metadata should be considered as a part of data. I'm not saying it is not a metadata; it is a metadata inside some data, so doesn't count for our purpose of defining a composite. I also don't think tar hardlinks are metadata for our purpose, because it technically consists of the linked path instead of the file contents and the information that the file is a hardlink, where the former is clearly a data and the latter is a metadata used to reconstruct the original file system so should be considered as a part of larger data (in this case, a logical notion of "file"). I believe these examples should be enough to derive my own definition of "composite". Please let me know otherwise.