Switches between the bit-depth of the modulation section of the algorithm and 32-bit float. The original hardware’s modulation section was only 8-bit, therefore when the modulation truncation setting is enabled the result is a grittier reverb with a richer top-end. This was one of the unique and familiar characteristics of the original device.
By turning off the 18-bit, Saturation and Mod. Truncation settings, you get a cleaner, more modern reverb.
Remember, in doing that, you sacrifice the edgy artefacts that gave the original hardware its unique “forward” sound.