Multispark CD systems fire the plugs mutiple times (upto 6) upto around 3000rpm, when there is not enough time to fire them more than once.

Capacitor Discharge ignitions (CDI, Multispark CDI - MSD 6AL, Crane Hi-6 etc) dont rely on charging the coil like traditional inductive systems, they have a capacitor that is charged upto around 450v, and this is dumped into the coil when the system is triggered, therefore all the dwell angles etc no longer mean anything.

Also with high compression ratio's, or forced induction setups, due to the high cylinder pressures, the spark can be "blown out" - hence sometimes the recommendation for closing the plug gaps.
CDI systems overcome this, because they produce a much more powerful spark, whick will be able to jump a 40 thou gap at these high pressires.

CDI systems can handle upto 10,000rpm, even using a standard coil.
Standard HEI systems, as in most falcons and commodores etc will start to reach their limits at 6000 rpm. There is simply not enough time to charge the coil/s at this speed.