My Jeep has a Civil Defense tag. I am a licensed radio amateur (ham), and I am registered with my local county to possibly help with emergency communications, should the need arise. Florida has four permanent issue front tags for emergency personnel:

PO - for Emergency Medical Technician or Paramedic

EMS - for Firefighter

FD - for Law Enforcement Officer

CD - for Emergency Management Personnel
The tag costs $5.00 Registration Tax + $2.05 = $7.05
Documentation and payment must be mailed directly to Tallahassee.

It is a real tag, intended to help you be recognized in an emergency. It does not expire, but you must return it to the state tag office, when you are no longer considered an emergency personnel. See page 7 of this December 2006 brochure.
Florida License Plates brochure
Florida Application for Front End License Plates
Here are pictures of my Jeep's tags:






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