All about U2
Most block coils without integrated ignition module have a low voltage connector with three pins: Battery voltage, 2x ignition signal (one for each coil).
Block coils with integrated ignition modules have more pins, for example U2003 has a four pin connector (battery voltage (12V), ground, 2x ignition signal (Trig) from ECM); the pin assignment generally depends of the car manufacturer.
U2 category block coils have at least four high voltage connections. Bigger coils can have up to 6 high voltage output connections. The ignition wires connect directly to the spark plugs.
With 4-Cylinder engines, the assignment is pairs of:
Cylinder 1 and 4
Cylinder 2 and 3
The wires of one cylinder pair can sometimes be interchanged.
Some engines use cylinder specific knock control, if wires are interchanged it can lead to misfiring.
In case of doubt refer to the mounting instruction in the box.
Example of configurations U 2001 (wires interchangeable):
Examples of configurations U 2003 (wires not interchangeable):
6 Cylinder engines
With 6- Cylinder engines, the assignment of the cylinder pair depends on the layout of the engine (firing sequence) , e.g." in line" or "V-Engine". Therefore no general assignation can be made.
Here is an example of the cylinder assignment of the coil U 2029: