"Caching" means that RealFlow | Cinema 4D writes the simulation data to disk (one file per fluid/mesh node and frame) and saves them for later use, e.g. for replaying the simulation or exchange with other programs.
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When enabled RealFlow | Cinema 4D will read existing cache files from the "Cache Folder (Mac OS X | Windows)".
Cache Simulation
A When you click on this button performs the simulation "offline". This means that you will see a progress bar, but the particles and/or meshes will not be visible in Cinema 4D's viewport. After the completion of the simulation "Use Cache" is enabled and you can play back the resultRealFlow | Cinema 4D will simulate the adjusted frame range. During this process, particles are not visible, but a progress bar informs you about the simulation's state and with “Cancel” the process is interrupted.
Remove Cache
Be careful with this button, because a click on it will remove all simulation files and empties the cache folder. This action cannot be undone!