Data items in the DATABASE_2 category record details about the
               database identifiers of the data block.
               These data items are assigned by database managers and should
               only appear in a data block if they originate from that source.
               The name of this category, DATABASE_2, arose because the
               category name DATABASE was already in use in the core CIF
               dictionary, but was used differently from the way it needed
               to be used in the mmCIF dictionary. Since CIF data names
               cannot be changed once they have been adopted, a new category
               had to be created.