/sys$common/syshlp/helplib.hlb MERGE, Specification File Qualifiers, /DATA, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    A /DATA qualifier must identify every field in the records you
    are directing to the output file. Specify the data fields in the
    order you want them to appear in the output record. By default,
    the record format for an output file is the same as that for
    the input file. If you want to eliminate or reorder fields from
    the output record, you can use the /DATA qualifier, causing only
    those fields identified by the /DATA qualifier to be directed to
    the output file.

    You can conditionally change the contents of a field in the
    output records by first specifying a condition name and then
    setting up a test for what meets the condition in a /CONDITION
    qualifier. You then specify the contents you want in the field in
    a /DATA qualifier of the form:

    /DATA=(IF condition-name THEN "new-contents" ELSE "new-contents")