By default, QlikView considers NULL values to be missing or undefined entities. However, certain database contexts imply that NULL values are to be considered as special values rather than simply missing values. The fact that NULL values are normally not allowed to link to other NULL values can be suspended by means of the NullAsValue statement.
In Pivot and Straight table the meaning is same for both
Null Symbol The symbol entered here will be used for displaying NULL values in the table.
Missing Symbol The symbol entered here will be used for displaying missing values in the
You see the refrence manual or helps in qlikview for more details
hope it helps