Quantity.
The units dispensed within an item: tablets, millilitres, grams. Useful, and easy to aggregate into nonsense.
Quantity records how much was dispensed for an item, in whatever unit the product is measured in: 56 tablets, 200 millilitres, one inhaler. It only means something within a single presentation.
Add tablet counts across strengths and you have a number with no meaning; 28 tablets of 20mg and 28 tablets of 40mg are not 56 of anything. The honest routes are per-presentation analysis, milligram conversion where the data supports it, or defined daily doses when comparing across drugs.
Items (prescription items) · One item is one product on one prescription form, dispensed once. It is the unit of NHS prescribing volume, and it is not a patient.
Defined daily dose (DDD) · The WHO's assumed average adult maintenance dose, used to compare prescribing volumes across drugs and strengths.
See it on a real molecule.
Every molecule page uses this term somewhere: in the scorecard, the charts or the honesty strip. The definitions live here; the numbers live there.