You are correct: the last digit tells you what type of apparatus it is.
0 - Pumper
1 - Pumper
2 - Aerial
3 - Tanker
4 - Rescue Pumper (i.e. pumper equipped with vehicle extrication tools)
5 - Rescue Quint
6 - Rescue Truck
7 - Ambulance
8 - Brush fire rig
9 - Utility vehicle
Now, if the 3rd digit is a 9, then it's some type of reserve vehicle
Examples: 2394 means it's Creve Coeur Fire District's reserve pumper with vehicle extrication tools
2597 means it's Brentwood Fire Department's reserve ambulance
0 - Pumper
1 - Pumper
2 - Aerial
3 - Tanker
4 - Rescue Pumper (i.e. pumper equipped with vehicle extrication tools)
5 - Rescue Quint
6 - Rescue Truck
7 - Ambulance
8 - Brush fire rig
9 - Utility vehicle
Now, if the 3rd digit is a 9, then it's some type of reserve vehicle
Examples: 2394 means it's Creve Coeur Fire District's reserve pumper with vehicle extrication tools
2597 means it's Brentwood Fire Department's reserve ambulance