The Plan allows for an expedited response for residents when calling for an ambulance, as 9-1-1's Computer Aided Dispatch (CAD) system chooses the next closest available agency when a primary unit isn't available, which expedites patient care. At this time, the distance based on the responding unit's station location to the municipality requiring a response.
The municipality that you reside in, (a City, Boro or Township) names who the primary responder is for BLS and/or ALS response. When that agency is not available, 9-1-1 has to send a "mutual aid" unit. By using the next closest available unit, it eliminates any issue of where the response is not set up in the best interests of the residents who dial 9-1-1 for help.