Most of us are familiar with voluntary consensus standards (VCS) but what makes a standard a mandatory standard? Well, that depends! This article will explore the different ways standards become mandatory standards and how you can determine whether a standard is mandatory.
The most common way for a standard to become mandatory is for it to be referenced in a regulation or law. For example, in Europe, many of the European Commission Directives (e.g. legislation) require adherence to “harmonized standards” to meet the requirements of the Directive. Good examples of these include those under the New Legislative Framework , adopted in 2008, to improve the internal market for goods and strengthen the conditions for placing a wide range of products on the EU market. This includes product legislation for items such as toys, medical devices, personal protective equipment, pressure equipment, construction products, radio equipment, etc. The Directives under this framework, set out requirements for conformity with harmonized standards, thereby making them mandatory. The list of compliant standards is updated frequently and published in the Official Journal of the European Commission.
Many other countries’ documentary standards are also required by law. Mexico and China designate their mandatory standards by the standards prefix. In Mexico, NOMs are mandatory (and are published in Diario Official), whereas the NMXs are voluntary. In China, any standard starting with GB is a mandatory national standard. GB/T standards are considered Voluntary National Standards and GB/Z standards are National Guiding Technical Documents.  Other countries where standards are also laws include Indonesia, Israel, Thailand, and Trinidad and Tobago.
In the United States, standards also become mandatory when they are “Incorporated by Reference” into regulations in the Code of Federal Regulations (CFR). An agency can incorporate part or the entire standard into a regulation making it required by law. Incorporation by reference (IBR) allows Federal agencies to comply with the requirement to publish rules in the Federal Register and the Code of Federal Regulations (CFR) by referring to material already published elsewhere. This makes IBR an efficient way for agencies to maximize their reliance on voluntary consensus standards and minimize their reliance on government-unique standards. For more information on the rules surrounding IBR, check out the National Archives and Records Administration’s (NARA) IBR Handbook updated in 2018.  The National Institute of Standards and Technology (NIST) also maintains a database of IBRs.  (Note: The database has not been updated since August 16, 2016 and is being provided as a source for historical data. Work is underway to bring the contents up-to-date and to maintain currency.)
The American National Standards Institute (ANSI), the official U.S. representative to the International Organization for Standardization (ISO) and, via the U.S. National Committee, the International Electrotechnical Commission (IEC), provides an IBR Portal site that links to standards developing organizations (SDOs). Here ANSI hosts full-text standards for read-only access. The site also provides links to the SDOs that hosts their own standards that have been incorporated by reference. 
Another way for standards to become mandatory in the US, is through State adoption of Uniform Laws or “model codes.” These codes spell out compliance requirements for specific standards. This is very prevalent in the building and construction industries. The standards are developed by a standards organization and local governments or councils can choose to adopt these codes as they see fit. The most popular of these include, AISC, the American Institute of Steel Construction, IAPMO, International Association of Plumbing and Mechanical Officials, and ICC, the International Code Council.
A standard that is commonly referenced in state and local requirements is NFPA 70®: National Electrical Code®. The code is adopted in all 50 states and sets the benchmark for safe electrical design, installation, and inspection in those states. Another term for this is “De jure standards”, or standards established by law.
Conversely, “De facto standards” are standards that are so commonly used they’ve achieved public acceptance as the ‘normal way of doing things.’ These standards may be adopted widely by an industry and its customers. They are also known as market-driven standards. These standards arise when a critical mass simply likes them well enough to collectively use them. Market-driven standards can become de jure standards if they are adopted into law. A great example of this is ASTM F963, Standard Consumer Safety Specification for Toy Safety. For the longest time, F963 was the “industry standard” for toy safety. The standard was continually updated and maintained to keep pace with innovation, include new information, and ensure consistent application. Almost every single manufacturer complied with F963 and most retailers wouldn’t purchase toys to sell or distribute unless they did. While not mandatory, for all intents and purposes, if you didn’t comply with this de facto standard, you most likely were not going to be in the toy market for long. In 2009, the standard was incorporated by reference by the Consumer Product Safety Commission (CPSC) requiring that all toys must meet the safety requirements of F963, per the Consumer Product Safety Improvement Act of 2008 (CPSIA).
This brings up the last category of “mandatory” standards. Standards that a manufacturer is required to comply with because the purchaser (retailer or distributor) says so. Most major retailers have strict quality and product integrity programs in place to ensure that products meet safety and performance standards. This is to help prevent recalls and product safety issues. These companies may require a manufacturer to meet industry standards that are voluntary or their own product specifications.
In summary, the requirement to comply with a standard can depend on the country, state, regulatory system, standards developer, industry, general acceptance of the standard, and other factors. So, when you ask what makes a standard mandatory the answer is, well, that depends. Hopefully, this article has provided you with a greater understanding of the varying factors that might make a standard a requirement. European Commission. New legislative framework.  PRC Standards System: Standards Used in China.  National Archives and Records Administration. IBR Handbook.  NIST. Standard Incorporated by Reference Database.  ANSI. IBR Portal Standards Incorporated by Reference.