It is file that describes a text format in which exiftool should write its output. To use this or other format file with exiftool you should save it somewhere on your disk (prefer pathes without spaces), and then add
-p "X:\Path\To\Your\Format\File\exif.fmt" to the arguments when launch exiftool application. In Windows that could be done via a .bat file or a shortcut .lnk or directly in the command prompt.
You can read more about -p key here: https://exiftool.org/exiftool_pod.html#p-FMTFILE-or-STR--printFormat
My orignal purpose is to use it with ImageGlass image viewer while working in Stable Diffusion UI. AUTOMATIC1111/stable-diffusion-webui#1569 (comment)
For some questions and answers look at the issue #1 .