Category holds links to other external data sources for the I/H model entry.
   Input datasets held in other databases such as EMDB, BMRB, SASBDB etc. 
   are referenced in the IHM_DATASET_RELATED_DB_REFERENCE category. 
   This data category, along with IHM_EXTERNAL_FILES category, holds information
   regarding other non-database external data sources, such as  DOIs (digital 
   object identifiers) or supplementary files stored locally. The DOIs can either 
   lead to the external data file(s) directly (as in case of DOIs provided by the PDB) 
   or might lead to an HTML landing page (as provided by Zenodo). In the latter case, 
   additional URL (Uniform Resource Locator) information is required to retrieve 
   the external data file(s).