Whether net mass or supplementary unit is required depends on the commodity code used. You only need to complete one of these two data fields.
Net mass (weight including immediate packaging) is collected for most goods because it refers to the physical characteristics of goods and is free from valuation problems. Very often the weight of goods is a more reliable indicator of goods movement than the value.
Net mass, if required, is shown in kilograms rounded up to the nearest kilogram. Only show the figure, not the 'kg' letters.
You are required to declare the supplementary unit for certain commodity codes the Intrastat Classification Nomenclature (ICN) online will indicate if this is required. Where a supplementary unit is requested, you will have to input the required information.
If a supplementary unit is required, you do not have to supply a net mass figure.