files-to-dump is either a mountpoint of a filesystem or a list of files
     and directories to be backed up as a subset of a filesystem.  In the for-
     mer case, either the path to a mounted filesystem or the device of an
     unmounted filesystem can be used.  In the latter case, certain restric-
     tions are placed on the backup: -u is not allowed, the only dump level
     that is supported is -0 and all the files and directories must reside on
     the same filesystem.

Thanks,
Fran

You need to use GNU tar (be sure its 1.13.19 or greater) to do incrementals
of subdirectories.  Some dumps only do entire filesystems, and some
will let you do full dumps of a subdirectory, but I don't think any will
do incrementals of one.

frank