The matrix field on a UnitConversion is important because the same numeric units can represent different concentration scales in different media. A conversion valid for groundwater concentrations (typically in ug/L or mg/L) may not be appropriate for soil concentrations (typically in mg/kg). By filtering UnitConversion lookups by matrix, piper selects the correct factor when normalizing results. When matrix is null or omitted, the conversion is treated as matrix-agnostic and applies to all media.
Technical section
Data model, API, and internals — visible to developers.