The rundown is, if you have a mission that has either 20 or 25+ patients (I forget which) you will need two trained “EMS Command” personnel on scene. They can arrive in either the EMS chief or the dedicated EMS mobile command vehicle. The difference being is only the EMS Command vehicle has the option to transport patients off the scene with many cool features you can set to your liking.
The EMS Support as stated will just up the process of the mission.