If they are grading a in-house built apparatus, rules are a bit different. Commonly Tankers are home-built more than anything else, followed by Squads/Brush units. This depends on the call load breakdown of the department. What do they use more? Tankers are somewhat universal since you can use them anywhere when you need water, but a Brush Truck may not always go to the house fire. The basic rule of thumb is getting your specs down on paper.
Maximum Age for Apparatus
The Fire Suppression Rating Schedule (FSRS) does not specify any maximum age for fire apparatus. ISO uses NFPA standards to define whether or not the apparatus meets general requirements. In addition, pumper apparatus must pass a pump test. ISO will credit a pumper with its capability (gallons per minute) at a net pump pressure of 150 psi. Aerial apparatus must also pass an annual service test (including a nondestructive test at least every five years).