The BLANK WHEN ZERO field attribute clause sets an  entire  field  to
 blanks  when  you  assign  it  a  zero.  Only VAX COBOL supports this
 feature.  All other processors ignore it.

Additional information available:

Example

 In the following example, the field NEW is initially  set  to  blanks
 when it is accessed by the VAX COBOL compiler.

 ZIP_CODE STRUCTURE.
     NEW        DATATYPE IS UNSIGNED NUMERIC
                SIZE IS 4 DIGITS
                BLANK WHEN ZERO.
     OLD        DATATYPE IS UNSIGNED NUMERIC
                SIZE IS FIVE DIGITS.
 END ZIP_CODE STRUCTURE.