Certain media formats can contain other entire media formats fully embedded.
I believe some image formats contain thumbnail images which are themselves full image files rather than just the bitmaps. Another example is that MP3 files can include full image files embedded within them for things such as album art. Those image files seem to be able to have their usual kinds of metadata in them.
I know Exiftool can recursively scan metadata formats within metadata formats, such as XMP within Jpeg Exif or MP3 ID3 metadata.
But so far I can't find a way to get it to also recursively scan the metadata of embedded files.
Am I missing something? Is there a way to do this? If not I'll go ahead and file a feature request (-: