Interesting, I just reviewed the application for the Florida Board of Engineers.
If you are looking for a WORK REFERENCE, that person needed to be overseeing/responsible for the work you were conducting.
If you are looking for a PERSONAL REFERENCE, then that person only needs to be a licensed engineer that will attest (based on thier knowledge) that you have been engaged in the practice of engineering for xx years and that you have accepted increasing responsibility over xx years.
In conclusion, a personal reference does not seem to discriminate between engineering disciplines, but a work reference would have needed to be engaged in the work that you were conducting.
JR