In certain circumstances it might be a business requirement to have a case participant, other than the primary case participant, stored as a piece of evidence data. A simple example of this would be for a piece of evidence named 'Medical Report'. Not only is it necessary to store the ID of the person for whom the medical report was commissioned, it's also necessary to store the ID of the medical practitioner who compiled the report. Other examples of case participants might be Education Faculties, Unions or Employers.
As the above scenario has in the past caused difficulties for developers, a new feature now exists which allows a developer to flag an attribute, via metadata, as being a special 'case participant' attribute. This means that this attribute will store the role ID of the case participant. The developer must provide the name attribute in the CaseParticipant element of the EUIM metadata, and use this name when aggregating the structs.