If this config option is enabled then overlay filesystems will use
the index directory to decode overlay NFS file handles by default.
In this case, it is still possible to turn off NFS export support
globally with the "nfs_export=off" module option or on a filesystem
instance basis with the "nfs_export=off" mount option.

The NFS export feature creates an index on copy up of every file and
directory. This full index is used to detect overlay filesystems
inconsistencies on lookup, like redirect from multiple upper dirs to
the same lower dir. The full index may incur some overhead on mount
time, especially when verifying that directory file handles are not
stale.

Note, that the NFS export feature is not backward compatible.
That is, mounting an overlay which has a full index on a kernel
that doesn't support this feature will have unexpected results.

Most users should say N here and enable this feature on a case-by-
case basis with the "nfs_export=on" mount option.

Say N unless you fully understand the consequences.