Body frame size (small/medium/large) from your height-to-wrist-circumference ratio.
It divides your height by your wrist circumference (both in cm) and compares the ratio against sex-specific thresholds — a higher ratio (relatively thin wrist for your height) means a smaller frame.
Men and women have different typical wrist-to-height proportions, so the calculator uses separate cutoffs: for men, >10.4 is small and <9.6 is large; for women, >11 is small and <10.1 is large.
It's a rough classifier (small/medium/large) sometimes used alongside height-weight charts to account for differences in bone structure — it doesn't measure body fat or muscle mass.