To expand upon nozzlenut's response:
EMD is kind of like a 'Choose Your Own Adventure" book. When a person calls 911 for a medical emergency and the center utilizes EMD, the calltaker determines the type of medical emergency. That determines the card number they go to in the flip file:
1 Abdominal Pain
2 Allergic Reaction
3 Animal Bite
4 Assault
5 Back Pain
6 Breathing Difficulty
7 Burns
8 CO/HazMat/Inhallation Injury
9 Cardiac Arrest/Respiratory Arrest/DOA
10 Chest Pain
11 Choking
12 Convulsions/Seizure
13 Diabetic
14 Drowning
15 Electrocution/Lightning
16 Eye Injury
17 Falls
18 Headache
19 Heart Problems
20 Heat/Cold Emergency
21 Hemorrhaging
22 Inaccessible/Machine Entrapment/Other Entrapments
23 Overdose/Poisoning
24 OB/Childbirth
25 Psychiatric/Suicidal/Abnormal Behavior
26 Sick Person/General Illness
27 Stabbing/Gunshot/Penetrating Trauma
28 Stroke/CVA
29 Motor-Vehicle Collision
30 Traumatic Injury
31 Unconscious
32 Unknown Situation
33 Transfers
Once they get to the correct card number, there are additional questions to ask that determine the appropriate response. One recommended format is as follows:
Alpha: BLS only, no lights/siren
Bravo: BLS only, lights/siren
Charlie: BLS lights/siren, ALS no lights/siren
Delta: BLS and ALS lights/siren
Echo: Multi-agency response... ambulance with ALS and possibly FD, PD defib run, chopper, etc.
Omega: Referral to another agency
And then other special circumstances or determinants are noted with the numbers 1-5.
So a final determinant code may be 29-B-1 for a minor-injury MVA (as above) indicating a BLS-only emergency response. a 9-E-1 will get an ambulance and ALS, and likely the FD and PD, depending on your area's protocols. Where I am, the sheriff's department has outfitted all of their cars with AEDs. According to their dispatch protocols, an Echo level EMS call takes priority over ALL police jobs except for felony 'in progress' jobs. You may actually be hearing "23-Delta-3" with is an ALS level poisoning or OD call.
Now, every 911 center has the prerogative to tailor the protocols to their own individual area of responsibility. Don't take what I've said here as gospel, but rather a guideline.