This category redefinition has been added to extend the key of
    the standard DIFFRN_REFLN category.

    Data items in the DIFFRN_REFLN category record details about
    the intensities in the diffraction data set
    identified by _diffrn_refln.diffrn_id.

    The DIFFRN_REFLN data items refer to individual intensity
    measurements and must be included in looped lists.

    The DIFFRN_REFLNS data items specify the parameters that apply
    to all intensity  measurements in the particular diffraction
    data set identified by _diffrn_reflns.diffrn_id and
    _diffrn_refln.frame_id