The carrier charges for whatever takes more of its capacity: weight or space. It therefore compares gross weight with volumetric weight and invoices the greater. A light, bulky shipment pays by volume; a dense one pays by weight.
Each mode has its convention: IATA air uses 1:6000 (167 kg/m³), express integrators 1:5000, sea LCL charges per tonne or cubic metre (W/M) at 1 t = 1 m³, and road by loading metre (about 1,750 kg per LDM) or density.
These ratios are market conventions, not law: a contract may set different ones. The chargeable-weight tool applies the usual ones and lets you change the factor.