There are two types of HID Projectors available. The first one is a single Xenon projector and the other one is a Bi-Xenon projector. The difference between the two is that one has a Solenoid attached to the bottom or on either side of the HID projector. Sometimes it has a Solenoid attached in front of the Shield.
Here is a clear image of a Single and Bi-Xenon HID Projector: