Remote File Sharing (RFS) was a distributed file system developed by AT&T in the 1980s. It was first delivered with UNIX System V Release 3 (SVR3). It was also included in AT&T's UNIX System V Release 4, but as that also included NFS which was more interoperable, RFS was little used. Some licensees of AT&T's UNIX System V Release 4 didn't include the RFS support in their SVR4 distributions, and Sun Microsystems removed it from Solaris 2.4.
Compared to NFS it made quite different design decisions. Instead of focusing on reliable operation in the presence of failures, it focused on preserving UNIX file system semantics across the network. Unlike NFS (before version 4), the RFS server maintains state to keep track of how many times a file has been opened, if any process has locked the file, etc. RFS was a product from Bell Laboratories.