A technical standard is an established norm or requirement in regard to technical systems. It is usually a formal document that establishes uniform engineering or technical criteria, methods, processes and practices. In contrast, a custom, convention, company product, corporate standard, etc. that becomes generally accepted and dominant is often called a de facto standard. A technical standard can also be a controlled artifact or similar formal means used for calibration. Reference standards and certified reference materials have an assigned value by direct comparison with a reference base. A primary standard is a technical standard which is not subordinate to any other standard but serves to define the property in question. Primary standards are usually kept in the custody of a national standards body. A hierarchy of secondary, tertiary, and check standards are calibrated by comparison to the primary standard; only those on the lowest level are used for actual measurement work in a metrology system. A key requirement in this case is (metrological) traceability, an unbroken paper trail of calibrations back to the primary standard. A technical standard may be developed privately or unilaterally, for example by a corporation, regulatory body, military, etc. Standards can also be developed by groups such as trade unions, and trade associations. Standards organizations often have more diverse input and usually develop voluntary standards: these might become mandatory if adopted by a government, business contract, etc. The standardization process may be by edict or may involve the formal consensus of technical experts.