some subs have 1 voice coil, some 2, some 4. it refers to the how many sets of coils are being used. singles have 1, duals have 2, quads have 4. it ussuall all comes down to haveing flexibility with your wiring options. with dual and quad voice coils, u can wire each sub up to a few differant impedience loads than if you had a single oice coil where youd be stuck at that one given load that that single voice coil reads.
only thing is you HAVE to have powe ron all the coils, cant power just 1 of the 2 or 4 coils, if one isnt getting equal power itll **** the sub up, and quickly too...
in the end, its mostly imp load flexibility so u can match ur subs to ur amps better