T o understand whether a product is truly a plant biostimulant, look at the evidence of its effects when used as recommended, which means taking into account the relative concentration of ingredients, dosage, timing, application method, target crops and other parameters. Under the EU Fertilising Products Regulation (2019/1009) , producers must demonstrate that their products have a justified plant biostimulant effect to place them on the market. The Committee for European Standardisation (CEN) is currently elaborating standards on how to demonstrate that biostimulant claims are justified.Regulation (EU) 2019/1009 defines seven Product Function Categories, including Plant Biostimulants, “by virtue of their claimed function” (Annex I, Part I, para. 1). The regulation also notes that those claims may be supported by “mode of action, the relative content of its various components, or any other relevant parameter” (Annex I, Part I, para. 3). For example, factors that determine whether products containing copper compounds are fertilisers or fungicides include the target crops, timing in the growth cycle, application rates and methods, etc. Similarly, while biostimulants may contain some nutrient elements, the amounts are usually too small to have a fertilising effect.
Researchers have long noted that “Biostimulants are defined by what they do more than by what they are, since the category includes a diversity of substances.” (Zhang and Schmidt 1997  quoted by du Jardin 2012 )
While there is general agreement about this principle, many people still confuse specific products with individual ingredients. But a single ingredient does not define a product. The same ingredient may be in products with extremely different functions. Also, combinations do not necessarily keep the characteristics of their components. Increasingly, plant biostimulants are complex formulations combining different ingredients because the combinations trigger system effects  that are more valuable to farmers than any of the individual components alone.