VMS Help MERGE, Specification File Qualifiers, /WORK_FILES, Qualifier Value  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 device

    Specifies a logical name for the work file. Unlike the DCL
    qualifier /WORK_FILES=n, the specification file qualifier /WORK_
    FILES=(device[,...]) specifies work file assignments, not the
    number of work files.