VMS Help OPEN, Qualifiers, /APPEND  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Opens an existing file for writing and positions the record
    pointer at the end-of-file (EOF). New records are added to the
    end of the file.

    Only sequential files allow more than one user to append records
    concurrently.

    Use the /APPEND qualifier only to add records to an existing
    file. The /APPEND and the /WRITE qualifiers are mutually
    exclusive.