Data items in the PHASING_SET category record details about
               the data sets used in a phasing experiment. A given data set
               may be used in a number of different ways; for instance, a
               single data set could be used both as an isomorphous derivative
               and as a component of a multi-wavelength calculation. This
               category establishes identifiers for each data set, and permits
               the archiving of a subset of experimental information for each
               data set (cell constants, wavelength, temperature, etc.)

               This and related categories of data items are provided so that
               derivative intensity and phase information can be stored in
               the same data block as the information for the refined
               structure.

               If it is deemed desirable to archive all possible experimental
               information for each data set (raw data sets, crystal growth
               conditions, etc.) those data items should be recorded in a
               separate data block.