Mass information needs to be provided for each of elements in a material declaration that correponds to a physical object or chemical (ie. product, product part, material, substance group, and substance in the figure below). The product element sits at the top of the declaration hierarchy therefore it must always include the mass (a mass percent would be meaningless). For the other elements, the responder who is creating a material declaration generally may provide either the mass or the mass percent.

Conceptual Material Declaration with Optional Declaration of Product Parts and Materials

Mass percent is calculated relative to the element above it in the material declaration. For example, the mass percent of material X is calculated relative to the mass of Product part 1 and the mass percent of Product part 1 is calculated relative to the mass of Product. An exception to the above rule is that the mass percent of substances and substance groups are both calculated relative to the material (or product part / product if material is not declared). A substance mass percent is never calculated relative to the mass of a substance group. This is because substance groups are often unusual grouping of substances and the mass may represent only a part of the substance molecules. For example, Lead/Lead Compounds only considers the Lead atoms and not the entire mass of lead compounds.

If the reporting threshold of a substance or substance group is based on the material (e.g. the RoHS substance thresholds), then the the MatMassPercent element must be used to capture the mass information.