MakerNote

Exif tag 37500, "MakerNote", is (unfortunately) a popular way store custom metadata.

By the Exif standard, it is merely a block of arbitrary bytes, which creators of Exif files may use in any way they like.

Some of the data formats used in MakerNotes have been documented or reverse-engineered. A MakerNote usually begins with a signature of some sort, such as the name of a camera manufacturer, which can help to identify its contents.