Tubes are typically sized in inches.. common is 1" OD. which is 7/8" ID
metric equivalents 25.4, and 22.2 are same thing..

Has to be threaded far enough to screw the top adjustable cup race down .

If course now , any threading is optional, since 1" threadless headsets are made ..

Italian is RH thread 36mm both BB sides.. a British BB thread is smaller than Italian
drive side is LH thread.