Denier measures the weight of a fibre per 9,000 metres of length — the higher the number, the heavier and denser the thread. 600D nylon sits at the practical upper end of everyday carry fabrics: dense enough to resist abrasion, puncture, and the slow attrition of zips, buckles, and rough surfaces, without the stiffness or excess weight of heavier industrial weaves. It holds structure without scaffolding, and takes wear without showing it.