All about U1
An NGK ignition coil in category U1 is almost always for vehicles with a mechanical ignition distributor and it supplies the spark plugs with high voltage via the distributor.
This diagram shows the high voltage distribution using a mechanical distributor:
A rotor arm attached to the top of the distributor shaft rotates underneath the distributor cap. As the distributor rotor passes the contact surfaces of the individual cylinders, the high voltage pulse is fed to the respective cylinders.