The "Frame Offset" parameter is part of RealFlow | Cinema 4D 2.1 and will be available soon.
"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.
...
When enabled RealFlow | Cinema 4D will read existing cache files from the "Cache Folder (macOS | Windows)".
Frame Offset
This parameter is only relevant with cached simulations and effects fluids, rigids, elastics, and meshes. You can specify a global frame offset, valid for all nodes. This global "Frame Offset" also affects the individual node offsets:
Total offset = frame offset + node-specific offset
Deformers Cache Mode
There are three options:
...