The FITS standard defines a format for metadata headers and mandates some of them.
Within reason, you can put pretty much anything in there. Whether a particular FITS-reading program interprets them in an intended manner is an entirely different matter.
For instance, whether either, neither, or both of “EXPOSURE” or “EXPTIME” is present depends on the writing program. My code to read FITS files has to be able to handle both and to work out what to do if their values differ.