The difference is the thread angle: ISO/English threads have a 60° angle, while Italian threads have a 55° angle. ISO/English and Italian are functionally interchangeable, but mismatch will slightly reform the threads, so it is discouraged to go back and forth between them. Once you've installed a mismatch, stick with it.

N.B. with an aluminum headset on a steel steer tube, it is the headset parts that will be reformed. With a steel headset on a steel steer tube, both the headset and the steer tube will suffer reforming.