This is the directory being exported to the clients listed in the Export to.. section.
Instead of mounting a number of distinct exports, an NFSv4 client sees the NFSv4 server's exports as existing inside a single filesystem, called the NFSv4 pseudofilesystem.
So for NFSv4, this directory is firsly mounted (with `mount --bind`) in the NFSv4 Pseudofilesystem, the pseudofilesystem is exported (if it wasn't), then the directory.
It is preferable to export this directory without hiding it, so the client will be able to move in it without mounting it.
Unlike other NFS servers, Linux supports the re-exporting of a directory that has been NFS mounted from another host, and the exporting of a directory that contains mount points for other filesystems.