Many people today are looking for improved lighting output from their car's stock halogen projector headlights. Quite often, the original halogen bulb used is an H11 style bulb. However, there are actually two different mounting styles for these H11 bulbs; H11A and H11B. The difference is in the orientation of the three little tabs around the circumference of the bulb's base.
The socket of the headlight housing will have three reliefs or cutouts for these tabs to be inserted. One tab is wider than the other two. It is the orientation of this widest slot on the headlight housing that will determine which bulb you will need. See the picture below for reference.
If you use the wrong bulb, the return wire on the HID bulb will be at the top once installed in the headlight. The return wire will cast a shadow down the middle of the car's light output, causing reduced performance of the light. It's not the end of the world, but you won't be getting the best light output possible.
The answer is simple. You need to look at the back of your headlight (though this is easier said than done sometimes). Either light will do, both will be the same. Note the orientation of the widest slot, then place your order accordingly. That's it!
For a more detailed explanation, check out this video from our friends at The Retrofit Source: