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