From Linux NFS
While NFSv2 and NFSv3 use a separate mount protocol to discover a server's exported filesystems, NFSv4 uses the same standard filesystem protocol (lookup, readdir, etc.) that is used to traverse within filesystems.
This gives the impression that these filesystems are all mounted on top of a top-level "pseudofilesystem".
Rather than constructing the pseudofilesystem from the list of exports in the /etc/exports file, the nfsd server just uses a real filesystem as the pseudofilesystem, and the administrator to export filesystems mounted underneath it. So that the server knows which exported filesystem to use as the pseudofilesystem (the filesystem that NFSv4 clients will see as "/"), that filesystem is marked with the export option "fsid=0".
This system was relatively simple to implement, but leads to severe problems for automount users, or for anyone attempt to migrate from NFSv2/v3 to v4, because v4 clients see different paths than mountd clients.