In Italian Suiting, the drop refers to the jacket waist measurement, different than in the US. The Italian Drop system goes from 0 (portly) to 8 (slim). As the drop number gets higher/ skinnier, the armhole also gets higher/ smaller making for a trimmer sleeve pipe. This is why Italian cut suits give the wearer a trim silhouette, even if you are a larger gentleman. Its built into the cut of the suit, and can actually make them look a lot trimmer than they actually are.
The Italian drop system is a really nice feature, since the waist measurement is completely separate from the chest, you don't have to bump up to the next chest size just because the fit doesnt work in the torso. This happens all the time in standard US boxy cut suits, if the torso is too tight they try to move you up a whole chest size to accomodate, which is why you see all tehse guys walking around with oversized linebacker shoulders on their jackets.
Hope this helps, and doesnt confuse the issue!