Lights wiring polarity and wire marking
White stripe is ground. Consistency is a virtue, and Schmidt's obvious decision tells us the industry default. In principle, the two conductors are interchangeable, because AC. However, in the reality of bicycles, there are practical considerations. Some dynamos ground one side of the circuit through the axle and some don't. Some lights ground one side through the mounting screws and some don't. Some bike frames are conductive and some aren't (and headset bearings are intermittent). So it is important for all deliberate connections to have compatible ideas of what is ground. Otherwise you have a short circuit. When a mechanic replaces your dynamo or light with a unit that makes an assumption about what's grounded, the assumption better match your wiring.