VMS Help MERGE, Output File Qualifiers, /BUCKET_SIZE, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Use the /BUCKET_SIZE qualifier with relative and indexed-
    sequential output disk files to specify OpenVMS RMS bucket size
    (the number of 512-byte blocks per bucket). If the output file
    organization is the same as for the input files, the default
    value is the same as the first input file bucket size. If output
    file organization is different, the default value is 1. The
    maximum number of blocks per bucket is 32.