Projectors with only low-beam output or 'single beam' are Halogen. You may put in 1 Ballast + 1 Bulb (bulb size depends on projector and your wiring harness, but it should have only one point) and achieve low beam ONLY from Halogen Projectors.
Projectors with high & low-beam output are Bi-Xenon. You may put 1 Ballast + 1 Bulb (bulb size depends on projector and your wiring harness, but it should have only one point) and achieve both high & low beam from Halogen Projectors.
Difference:
HID bulbs can have a singe or dual point in the bulbs. These can light up individually and collectively depending on headlight assy.
Projectors are capable of producing both high and low beams from 1 single bulb with 1 single point. Because there is a 'flap' in the projector that controls the throw of the bulb.
Hope it was not too confusing.
Cheers.