It basically says if there are no rows, hide the tablix. In general, visibility expressions are formatted "IF X, Do this, Otherwise do that." True means hide, False means don't hide. The format "=IFF X, True, False" means the default is to display the tablix, but only hide it if X. The format "=IFF X, False, True" means the default is to hide the tablix, and only show it if X.