From the CFMU ATFCM manual:
Hospital Flights
After consultation with the European Aero Medical Institute (EURAMI), flights which are categorized as NACA V or VI would always qualify for approval to use STS/ATFMEXEMPTAPPROVED.
Flights categorized, as NACA IV would normally be expected not to use
STS/ATFMEXEMPTAPPROVED. Should such a flight receive an ATFCM slot with a delay, which the medical personnel accompanying the patient, consider would jeopardize the patients ability to recover, they have been advised to contact the CFMU helpdesk. Such calls shall be treated sympathetically, and every effort made to reduce any delay to a minimum.
A flight positioning to an airport to collect a patient categorised as NACA V or NACA VI, and doing an immediate turnaround with the patient on board to return would also qualify for STS/ATFMEXEMPTAPPROVED. However, routine positioning flights e.g. returning empty after an evacuation or positioning for fuel, or positioning to an airport to collect a patient some time after arrival (i.e. not time-critical) do not qualify for the use of STS/ATFMEXEMPTAPPROVED.
Note: The NACA (National Advisory Committee For Aeronautics) scale is an international scoring system, ranging from 1 to 7 points, to give an overall description of a patient’s medical condition:
NACA I - minor health disturbance
NACA II - out-patient check up needed
NACA III - hospital treatment needed
NACA IV - possibly health threatening
NACA V - acute critical condition
NACA VI - resuscitation
NACA VII - death
A NACA score of III or more points includes severe disease or injury patterns, which may need further and urgent treatment in more specialised hospitals.
Where the CFMU receives a call from an aircraft operator requesting approval for an STS/ATFMEXEMPTAPPROVED, that caller shall be referred to the relevant national AIP to find the appropriate office, or FMP if such an office has not been nominated or is unavailable.
Hope this helps.
flowman