Dimensioning / cubing.

A gate, an overhead sensor or a static cube box measures a parcel's three dimensions as it moves, normally paired with an in-motion scale so cube and weight are captured in one pass. It decides the shipping bill, because carriers rate on whichever is greater, actual weight or dimensional weight, and they re-measure the parcel in their own hub and bill the difference back weeks later. In the United States a device used to work out freight, storage or postal charges is a legal-for-trade instrument under NIST Handbook 44, Section 5.58, Multiple Dimension Measuring Devices, which is why a dimensioner is sealed and certified and a tape measure on the packing bench is not. The failure specific to apparel POD is that a polybag has no fixed cube: a soft mailer measured flat on the belt and puffed up in a carrier hub produces a carrier adjustment nobody can reconstruct afterwards. A shop that does not dimension is not comparing quoted rate against billed rate at all, and that gap only ever moves one way.