Its part of how the state identifies resources in the states mutual aid system. The state is divided into 6 geographic regions and then each region is subdivided in multiple areas (a group of cities/departments). That being said here is a quick break down;
The 1st 3 letters identify the Department/county (if it starts with an X it’s a Strike Team consisting of mixed Departments)
The 1st number represents the states mutual aid region they are from.
The 2 nd number represents what area they are from in that region.
The last 2 numbers represent a block of numbers that area is assigned for spefic Departments in that area.
That’s it in a nut shell!