Returned when a field definition includes a NULLIF or MISSINGIF clause that uses the field mapping format. For instance:
field1 (1:5) NULLIF (1:2) == 'ab'
The field mapping format
cannot
be used with the NULLIF and MISSINGIF clauses. If you wish to use a subset of a field in these clauses, use the LEFT(), RIGHT(), or MID() functions to return the substring.