The ISO uses a grading system from 1 to 10 with 1 being the best and 10 representing a community without an effective fire service. Within this grading structure, a split class means that all properties within 1,000 feet of a water supply (usually a fire hydrant) are eligible for the first class (1-8).
Properties that are more than 1,000 feet from a water supply, but within five road miles of a fire station, are eligible for either a Class 9 alone or Class N/NX split classification, where the X signifies an area protected by a Class N fire department and the area lacks an adequate water supply. All properties more than five road miles from a fire station are Class 10. An example would be a 3/3X classification, where most of the properties are within 1,000 feet of a water supply and meet other fire standards (Class 3), and some of the community is more than 1,000 feet from a water supply but within five road miles of a fire station (Class 3X). The challenge to local government is to improve the fire service using the savings from an improved insurance rate classification.
The ISO rates community fire service in Tennessee. According to the Tennessee County Fire Handbook, “For areas without a public water supply system, the following flow rates must be achievable for a minimum of two hours:
The flow, measured in gallons per minute (gpm), must be established within 15 minutes from the alarm time.” 
 Tennessee County Fire Handbook, Kevin J. Lauer, County Technical Assistance Service.