text: Other bodies may define classes of reference material differently. WHO guidelines for biological reference materials  provide the terms: 
  * Reference standards: materials that are used as calibrators in assays
  * International biological measurement standard: a biological substance provided to enable the results of biological assay or immunological assay procedures to be expressed in the same way throughout the world
  * Secondary reference standards: Reference standards calibrated against and traceable to primary WHO materials and intended for use in routine tests
  * Reference reagent: a WHO reference standard, the activity of which is defined by WHO in terms of a unit
  For chemical substances some pharmacopoeias use the WHO terms
  * Primary chemical reference substance: a chemical reference substance  ... whose value is accepted without requiring comparison to another chemical substance.
  * Secondary chemical reference substance: substance whose characteristics are assigned and/or calibrated by comparison with a primary chemical reference substance.
  The United States National Institute of Standards and Technology (NIST) uses the trade marked term Standard Reference Material (SRM) to denote a certified reference material that satisfies additional NIST-specific criteria. In addition, commercial producers adhering to criteria and protocols defined by NIST may use the trademark  "NIST traceable reference material" to designate certified reference materials with a well-defined traceability linkage to existing NIST standards for chemical measurements.