This standard replaced ILD as a measure of firmness in polyfoam and is almost identical with a different name. It pre-compresses a material with 1 lb of weight to make up for surface variations and is a little bit more accurate ... although for all intents and purposes they are interchangeable. We mostly use ILD on this website since it is more commonly known although is is not as "up to date" in the world of polyfoam at least.