This do not have to be present in the input file and are marked as &optional in the record description.
Those can e.g. be used to create field values on the file in a predicate while reading a file - example:
Input::add_table([$source="input.log", $name="input", $idx=Idx, $val=Val, $destination=servers,
$pred(typ: Input::Event, left: Idx, right: Val) = { right$notb = !right$b; return T; }
