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.

1  –  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.