I'm by no means an expert on this but for Boeing and MD
, the construction number is sequential based on the order in which the planes were ordered and the line number or fuselage number tells you which number it was off the line(across a model type). Due to my inability to use English today, here's an example (numbers taken from airlinerlist.com):
When an airline orders a number of planes, the aircraft to be built are given consecutive construction numbers...Pan Am's 747 launch order was assigned the numbers 19637 to 19661. The frame designated 19637 (N731PA) was the 4th off the line, so it is 19637/4. The fifth off the line, TWA's N93101, was the first of a series of TWA orders, starting with frame 19667, so it is 19667/5. The first number does not always follow the same sequence as the second number...for example, Pan Am's N732PA, cn 19638, rolled off the line in the number three position, before 19637 did (for whatever reason). It is via the second number that one can tell, for example, the 500th 747 (22381/500, N4501Q for SAS). The construction number refers not just to 747s, but a frame's order among all Boeing frames. Airbus serials forgo all these complications and only consist of the line number.