The definition for each tier is as listed below:
Tier 1: Population Density <20 persons per sq mile, <1% of the area is developed, and <5% of the landuse is agriculture
Tier 2: Population Density <36 persons per sq miles, <2% of the area is developed, and < 5% of the landuse is agriculture.
Tier 3: Population Density is <64 persons per sq mile, <3% of the area is developed, and <5% of the landuse is agriculture.
Tier 4: Population Density is <90 persons per sq mile, <3% of the area is developed, and < 5% of the landuse is agriculture.
A complete description of the dataset may be found in The Land Conservation Plan for New Hampshire's Coastal Watersheds (section III) or in the excerpt "Coastal_Plan-App_D-2-Freshwater.pdf".