"Most low-beam headlamps are specifically designed for use on only one side of the road. Headlamps for use in left-traffic countries have low-beam headlamps that "dip to the left"; the light is distributed with a downward/leftward bias to show the driver the road and signs ahead without blinding oncoming traffic.

The headlights have a cutoff shield which allows them to higher to the right on left hand drive (LHD) (e.g American) or higher on the left for right hand drive (RHD) e.g. English vehicles. These are referred to as cut-off lines.
European E-Code or UN ECE specify a different pattern than the American SAE (DOT).

The DOT drop is about 0.4-0.5° or a little less than 1%

Source: HOW TO: aim HID headlights - HiDplanet : The Official Automotive Lighting Forum

Test procedures:

1. Position vehicle on a level surface perpendicular to a flat wall 25 feet away from front of headlamp lens .
2. If necessary, tape a line on the floor 25 feet away from and parallel to the wall.
3. Place 165 lbs in the drivers seat to simulate the ride height of the vehicle when driven and gas tank is full
4. Measure from the floor up 4 ft and tape a line on the wall at the centerline of the vehicle. Sight along the centerline of the vehicle (from rear of vehicle forward) to verify accuracy of the line placement.
5. Rock vehicle side-to-side three times to allow suspension to stabilize.
6. Jounce front suspension three times by pushing downward on front bumper and releasing.
7. Measure the distance from the center of headlamp lens to the floor. Transfer measurement to the alignment screen (with tape). Use this line for up/down adjustment reference.
8. Place a tape line 2.5 inches below and parallel to the center of headlamp line.
9. Measure distance from the centerline of the vehicle to the center of each headlamp being aligned. Transfer measurements to screen (with tape) to each side of vehicle centerline. Use these lines for left/ right adjustment reference.