Diastase is an amylase enzyme that bees add to nectar, and the diastase number is a measure of how much of that enzyme activity survives in the honey, reported on the Schade scale. Because the enzyme is destroyed by heat, the number works as a heat-damage indicator: a high result means the honey has not been cooked, a low result means it has been, or that it is old, or in some cases that the crop was naturally low in enzymes to begin with.
Council Directive 2001/110/EC sets the general minimum at not less than 8 on the Schade scale. It also recognises the natural-variation problem by allowing a diastase down to 3 for honeys with a naturally low enzyme content, but only where HMF is not more than 15 mg per kg. That conditional pairing is the useful part of the rule: a low diastase can be innocent, and the way the Directive separates innocent from damaged is by requiring the honey to be demonstrably fresh at the same time.
Which is why the number should never be read on its own. Diastase and HMF move in opposite directions under heat, so the pair together distinguishes the three explanations for a weak diastase result: naturally low enzyme crop, low HMF; ordinary ageing, moderate HMF; heat treatment, high HMF. Only the third is a quality failure.
For a buyer specifying honey, the practical instruction is to contract both parameters, require the extraction period on the certificate so ageing can be separated from heating, and be realistic about which honeys sit naturally low. Citrus and some other light monoflorals are known low-enzyme crops, and a rigid minimum of 8 applied without the HMF cross-check will reject perfectly sound honey while doing nothing to catch a well-managed heat treatment.