              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.)\
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               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.