I play as realistically as I comfortably can, and one thing that I try to do is separate my "Unit" ARRs (that is, individual vehicles) by service - Met Police, MOD Police, British Transport Police etc. I have always done this by setting a generic "IRV" ARR, and selecting all stations for that service under the "Stations" drop-down menu, as it states "When a building is selected, only vehicles of the corresponding building will be selected."
This was very time consuming, repetitive and when I created a new station for a particular service, I had to add it to all of my ARRs for that service so that the appropriate units would be selected.
I had been contemplating suggesting a feature to select any vehicle assigned to a dispatch centre instead, as my services are all assigned separately, when I spotted that dispatch centres are in fact listed in the same drop-down menu.
I have tested this and to my surprise, it works the way I imagined it would!
TLDR: Selecting a dispatch centre in the "Stations" drop-down menu of an ARR will allow your ARR to select any vehicles matching the ARR from that dispatch centre only. Handy feature I will now be implementing across all of my gameplay.