Heat flux in mW/mm² (miliwatts per square millimeter) is the amount of of watts per surface area of your coil wire. Get that too high and the coil will burn
juice. Too low and the coil will not vaporize much
juice per second.
Heat capacity is the amount of heat (energy, not temperature) required to raise the temperature of your coil mass a certain amount. The higher that number is at the same power level, the longer it will take to get the coil to operating temperature.
Hope this helps