FumeFX offers several simulation modes.
Default - This mode lets you to quickly simulate a low detail grid which can be further refined in Wavelet mode. If adding a Wavelet Turbulence generated detail, you can use it to create highly detailed simulation to which you can then incorporate further detail by using Fluid Mapping.
Initial State - This button starts the simulation from a snapshot – a selected frame of the simulation, whose initial conditions were previously saved to a simulation file (.fdc) or output file (.fxd). The file can be selected from the Floater Menu > File > Load initial state. If it hasn't been selected yet, a file selection dialog box will open, prompting you to select it.
If you are using an fxd file for the initial state make sure that such a file contains velocity channel.
Wavelet - To be able to use this mode, you need to have caches prepared by using Default simulation mode first. This mode will reload the Default caches, upscale simulation grid and use saved information to create additional detail.
Post - Post mode allows you to retime and optimize caches created in Default or Wavelet modes. Caches optimization allows excluding of channels that are no longer needed. This can be very useful for building .fxd caches with minimum data needed for rendering. Second option is to optimize grid dimensions which will result in a tightest possible container around smoke/fire. Using both methods, files sizes can significantly decrease and memory requirements will decrease, so you can expect faster rendering times.