Rhenium DOS can operate using Interrupt, BIOS and FOSSIL communications. The benefit of the FOSSIL interface is you can use X00, ADF or NetFoss with DOSBox or a 32bit Windows CMD.EXE window. The FOSSIL offloads the overhead of communication buffering and timing, allowing Rhenium to focus on the transfer protocols.
A FOSSIL drive is only required/used by the DOS version of Rhenium.