64 bit Wave Format (RF64) is multichannel file format for file sizes to exceed 4 GB. RF64 is BWF compatible.
Supported for input and output files (for uncompressed stereo and multichannel PCM only).
If size of output wav-file less 4GB, output file will have RIFF-format (32-bit wav).
If size of output wav-file exceed 4GB, output file will have RF64-format (64-bit wav).
It is automatical process (without user control).
See more:
EBU Recommendation R111-2007 - Multichannel use of the BWF audio file format (MBWF)