There are two key measurements that you need to know when ordering a bottom bracket.

The first is the width of your the bottom bracket shell on your bike's frame. You can measure that with an ordinary metric ruler but you have to be pretty precise. If it's 68 or 73mm, you have an English threaded bottom bracket. If it's 70mm, you have an Italian threaded bottom bracket.

The second is spindle length. Spindle length, with a very few exceptions, is determined by your crankset. If you are reusing the same crankset, just measure the old BB spindle. If you are installing a new crankset, it should come with specs. for spindle length. Sometimes a crankset will recommend spindle length as "110/113mm." If that's the case, measure the distance between your frame's rear dropouts. If the distance between the rear dropouts is 130mm, use a 110mm spindle length. If the distance between the rear dropouts is 135mm, use a 113mm spindle length.

One last thing, which should be obvious, is the that the end of the spindle has to match the hole in your crankset. If it's splined, the splines have to match. If it's square, there are two different square tapers that look almost the same. There are undoubtedly exceptions but generally if you match only a Campy crank with a Campy bottom bracket you'll be OK.