Each pawn uses a body of a set type that defines all their body parts and the parts' base health. Some animals share body types - for example a Cat and a Cougar both use QuadrupedAnimalWithPawsAndTail, but obviously it doesn't make sense for the cat to be just as durable as the cougar. Health Scale acts as a multiplier for the HP of every body part.
So a Cat has a health scale of 0.42 and a cougar has a health scale of 1.3, so the tail of a cat will have 32% the hit points of a cougar tail.