That makes sense - of course the one disconnect is that in us sizing the trousers are labelled so a suit in size 42 will usually have trousers with a labelled 36 waist (or 35 if the cut is more athletic.) If you don't know the drop on the Euro sized clothing then sometimes its a surprise. And then of course there is size inflation and differences in cuts. In order to successfully order things online it helps to have exact measurements, to know what you are ordering through experience with that manufacturer, and finally to have an easy return policy. Of course, nothing works better than trying it on first!The concept of drop is a bit artificial amongst Italian tailoring companies.
In principle, the concept is the same of UK/US sizing but applied to half-measures of chest and waist. For example: a size 50 (100 cm = 40'' chest) in a drop 6 will come with trousers having half waist measuring 50 - 6 = 44 cm, that is to say a total waist of 88 cm = about 34.5''.
In the practice the term drop is more often used to classify the overall fit of the jacket, in particular shoulder-chest-waist ration. This use is unfortunately not formalised and varies from maker to maker but as a guide:
drop 4 = stout figure/portly fit
drop 6 = regular figure/classic fit (normal shoulders, normal waist)
drop 7 = slim figure/tailored fit (narrow shoulders, narrow waist)
drop 8 = athletic figure/slim fit (wide shoulder, very narrow waist)
It is very difficult to give general rules as they change from maker to maker. Most houses label each of their pattern blocks by the drop number so this becomes more an indicator of fit than size in these cases.