Originally Posted by JamesNoBrakes

It usually means FAA medical has become aware of a condition that disqualifies you from holding that medical certificate. A letter is sent requesting you to surrender your certificate. If you do not do so, then they'll begin legal action to issue an order requiring you to surrender it, which you can usually fight at your own expense and with your own lawyer, but if a legitimate case, you'll be spending money for no reason and still lose your medical. That said, mistakes have been made before by medical. Also, I don't know if it works the same way in medical, but with airmen certificates, if revoked, you are prohibited from applying again for said certificate for a period of time, like a year. If surrendered, an airman can re-apply for the same certificate again the next day. Some people choose this over an airman certificate 44709 when a 44709 is ordered, because they realize they got real poor instruction and do not think they could pass a practical test. So they go back and retrain and retake the actual practical, rather than risk losing it on the 44709. Got a bit off-topic, but that is some of how surrendering works with airman certificates.