The Shore durometer hardness rating, named after Albert Ferdinand Shore, is a quantified scale to measure the hardness of polymers and elastomers (i.e., plastics or rubbers). In other words, Shore hardness is a measurement of a product's ability to resist a concentrated, pressing load. A car tire has a Shore Type D hardness rating of 15 while a hardhat has a hardness rating of 80.
In hard surface products (i.e., ceramic tile, porcelain tile, stone tile, etc.), movement joints may require a hard caulk. "Movement joints are filled with material that allows for contraction and expansion. For floor applications, urethane, neoprene, or polysulfide are most often used in traffic areas and silicone sometimes where traffic is not a concern. Traffic areas require a sealant with a shore hardness of 35 or greater," according to the Tile Council of North America (TCNA). The same hardness ratings apply when selecting a caulk for a concrete joint.
The Shore durometer hardness rating is used to measure rubber tile flooring. Johnsonite explains: The durometer hardness "test is performed to the requirements outlined in ASTM D-2240 and utilizes a hand-held instrument called a 'Shore' gauge. The gauge consists of a spring-loaded indentor and has a scale reading from 0 to 100. The indentor is applied to the surface of the test material and the hardness of the material is obtained from the reading on the scale. The higher the reading, the harder the material. Hardness testing is used primarily to classify materials and no simple relationship exists between hardness testing and indentation or any fundamental property of the material."
See Janka ball test for a hardness testing method for hardwood.