Generally speaking, there are three models of HID if we classify HID by the lighting way.  They are single beam; double beam and Hi/Lo beam. Now let's talk about how to catalogue it. Normally, the production cost of same group above is more or less the same.

Single beam, There is only one xenon bulb that gives out far or near light. The normal models are: H1,H3,H4-1,H7,H8,H9,H10,H11,H13-1,9004-1,9005,9006,9007-1,880,881,etc.

Double beam, There are two bulbs (halogen bulb + xenon bulb or two xenon bulbs). One gives out far light while one gives near light. Its also called as bi-xenon because the are two bulbs. The normal models are: H4-2, H13-2, 9004-2, 9007-2, etc.

Flexible beam, the magnet inside the base pulls the unique xenon bulb up or down to give out far or near light. It's also called as flexible kit because the bulb is flexible. The normal models are: H4-3，H13-3;9004-3,9007-3;etc.