The idea behind this utility is basically the same as with genext2fs, except that this generates FAT12/FAT16/FAT32 filesystems instead (where the specific variant depends on the size of the generated filesystem). The point behind this is that you don't need root privileges to use this utility, and that the resulting filesystem should always be exactly the same regardless of the environment in which it was generated.
-p PATH path to root directory
-o image.img output file name
-s 1GiB filesystem size
-r 16 additional reserved sectors to allocate
-d disable generation of long filenames
-h, --help print this help menu
(the -p, -o and -s options always need to be specified.)
Run cargo build --release (assuming that you have a Rust toolchain), and the newly-built genrsfatfs binary should appear in target/release.