those are all variations in the voice coils. if you need a certain load on your amp, then you would get the one that you can hook up to your amp. for example, if you need a 2 ohm load on your amp, then you would get the D4. if you need a 1 ohm load on your amp, then you would get the D2. if you need a 3 ohm load on your amp, which only JL amps are good about that(probabaly a few other i don't know of), then you would get the D6. now, if you plan on getting two subs and you wanted to wire them to a 1ohm load, then you would get the D4. 2 subs for a 2 ohm load, then you would get the D2. and i'm not sure with the D6. don't feel like doing math. lol
hope this clears things up a bit. sorry for the late post.