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

    Use the /FIELD qualifier to define input record fields to be used
    for a sort or merge operation or in a conditional evaluation, or
    whose order or format will change in the output record.

    Identify each field by specifying a name in the /FIELD qualifier,
    a constant value or the field position, and the size and data
    type of the field. Field names must be unique; no duplicate
    field names are allowed. You cannot use more than 255 key field
    definitions.

    Once the field-name has been specified in the /FIELD qualifier,
    it can be used in the /CONDITION, /KEY, and /DATA qualifiers.