For the sake of completeness:
Mission generation:
* Generating building is assigned to a dispatch center with its own coverage area => only buildings of this dispatch center count towards the minimum requirements!
* Generating building is not assigned to a dispatch center => all buildings of the entire account (independent of dispatch centers) count towards the minimum requirements.
* Generating building is assigned to a control centre without its own coverage area => All buildings of the entire account (independent of dispatch centers) count towards the minimum requirements.
Important information about patients:
Patients can appear if a rescue station or active ambulance extension is "nearby" (I think it was max. 15km, but haven't found a source on this yet, so no guarantee!). Dispatch centers and coverage areas seem to be irrelevant here.
Disabled dispatch centers:
If a dispatch center is disabled, the buildings assigned to it can no longer generate operations; the above-mentioned cases for minimum requirements are independent of this.