$ gemmi cif2mtz -h
Usage:
  gemmi cif2mtz [options] CIF_FILE MTZ_FILE
  gemmi cif2mtz [options] CIF_FILE --dir=DIRECTORY
Options:
  -h, --help               Print usage and exit.
  -V, --version            Print version and exit.
  -v, --verbose            Verbose output.
  -b NAME, --block=NAME    mmCIF block to convert, by name.
  -B INDEX                 mmCIF block to convert, by index (default: 1).
  --add CIF_FILE           Use additional input mmCIF file, first block.
  -l, --list               Dry run and list blocks in mmCIF file.
  -d DIR, --dir=NAME       Output directory.
  --spec=FILE              Conversion spec.
  --print-spec             Print default spec and exit.
  --title                  MTZ title.
  -H LINE, --history=LINE  Add a history line.
  -u, --unmerged           Write unmerged MTZ file(s).
  --sort                   Order reflections according to Miller indices.

First variant: converts the first block of CIF_FILE, or the block
specified with --block=NAME, to MTZ file with given name.

Second variant: converts each block of CIF_FILE to one MTZ file
(block-name.mtz) in the specified DIRECTORY.

If CIF_FILE is -, the input is read from stdin.

To convert data from multiple CIF files into one MTZ file use:
  gemmi cif2mtz [options] CIF1 --add CIF2 --add CIF3 MTZ_FILE
It's for special cases, e.g. when map coefficients are in separate files.
