The difference between ‘amount of substance’ and ‘quantities that are a number of entities’ and implications for units of measurement

The recent conference on “The Units, Symbols, and Terminology in the Physical Sciences in and for the Digital Era (D-UST)” highlighted the need to address the shared challenge of achieving consensus of standards and terminology for the digital age, in particular making data both human and machine readable. A key challenge is the presentation of ‘counting units’ – more properly known as units for quantities that are a number of entities. It was clear from some discussions at the conference that there is remaining confusion about the difference between ‘amount of substance’ and ‘quantities that are a number of entities’. This article attempts to delineate properly between these two different quantities, provide wider clarity on current best practice in relevant nomenclature, and explain the implications for units of measurement, expanding on comments made by the author at the conference. It also reflects on some of the latest thinking and recent progress made by the chemical metrology community in the representation of quantities that are a number of entities.