A phantom set element is there syntactically but it does not exist. This was used in the past to define an empty parameter statement. For example:
$phantom empty parameter x / empty.empty 0 /
Nowadays one should use the
$onompty dollar control option:
$onempty parameter x(*,*) / /;
The warning indicates that in there is a phantom set element
empty in the workfile, which does not really exist.