It is much obscured and confusing in the United States Tim.
In Europe that was the pioneer in legislation, the things are very simple:
- Low power, low speed e-bike where the motor can only work when the thing is pedalled -- is just a bike like any other
- High speed and/or high-power e-bike (with optional throttle) is a moped, equipped with numerous safety features, and requiring a plate number, insurance, wearing the helmet, and a driving license. It has to be Type Approved for class L1e-B.
In the United States, the federal legislature has introduced the Class system, the best implemented by the state of California:
Any e-bike may be assisted by a motor up to 750 W.
- Class 1: Assistance up to 20 mph, no throttle, the thing must be pedalled
- Class 2: Same as Class 1 but there is a throttle and you may pedal the bike or just ride on throttle
- Class 3: Pedal-assisted, up to 28 mph. The thing can only be ridden on roads.
Now, state and local legislatures have never made a consistent set of laws, so you may ride your e-bike here but be banned from there. (For instance, it is very hard to legally ride e-bike in most of U.S. National Parks, while any e-bike is allowed to ride in any European National Park). Many U.S. residents simply don't care about the whole class system ("Who will ever check me?")
Having said the above: Disregarding the U.S. status, e-bike Classes were meant as something that must not be changed on given e-bike.