You'll usuallu encounter this when selecting a bottom bracket for your bike where either English 68mm or Italian 70mm is used. Here's some notes from the internet :
"Italian bicycles are built to specific standards of threading and dimension. Most parts on Italian dimension bicycles are interchangeable with British/ISO dimensioned parts.
The major exception is the bottom bracket. Italian bottom brackets are 70 mm wide, as opposed to the usual 68 mm dimension of British/ISO and French bottom brackets. The cup diameter is also larger. Sometimes, bicycles which have damaged bottom-bracket threads are machined out to Italian size to eliminate the damaged threads. Italian bottom brackets, like the French, use a right-hand thread on both sides, so the fixed cup is prone to loosening up unless very securely tightened.
Italian threading is a curious mixture of metric and British. Diameters are specified in millimetres, but threads are in threads-per-inch! In addition, the thread angle is 55 degrees, like the obsolete British Whitworth system, rather than 60 degrees as with U.S. and metric threads.
Italian freewheel and headset threads are the same as British/ISO, except for the thread angle difference. They may be interchanged, but you should not go back-and-forth between Italian and British/ISO headsets. Italian freewheels are basically extinct, so the issue is moot there. Italian size handlebars/stems generally use a clamping diameter of 26 mm. Many high-end aftermarket drop handlebars and "road" stems also use this size even if not Italian made. "