Certificate of Weight (CoW) or Certificate of Quantity is certifying the weight of the shipment. The CoW usually comes from SGS or similar based on inspection of the shipment.
The methodology for certifying the weight will depend on the shipping mode. If the shipping mode is containers; then the measurement of weight is by weighing the containers before and after loading of the commodity into the container.
If the shipping mode is bulk shiploads; the measurement of weight is by weighing the bags or big bags. The Certificate of Weight will indicate if all units went through the measurement; or also the total weight is an estimate according to the samples and calculations.