These special classes are used within the software to further refine CIDOC CRM classes for the user interface.
All OpenAtlas classes can be mapped directly to the corresponding CIDOC CRM classes. For all available classes and properties of CIDOC CRM (version 7.1.2) see here.
Example: The CIDOC class E18 - Physical Thing “comprises all persistent physical items with a relatively stable form, human-made or natural” (for more information see CIDOC CRM documentation). In OpenAtlas E18 is used to represent place, stratigraphic unit, and feature, which are all physical things according to CIDOC CRM. To differentiate between those different entities, the corresponding OpenAtlas classes were created. While place, stratigraphic unit, and feature are all considered E18 and mapped to this CIDOC CRM class in the background of the application, the user interface displays the OpenAtlas class name for user’s convenience and to avoid confusion.
The same is true for bibliography, edition, external reference and file, which are all considered to be E31 - Documents in CIDOC CRM or the OpenAtlas classes source and source translation which correspond to the CIDOC class E33 - Linguistic object.
An overview of all OpenAtlas classes used and how they correspond to CIDOC CRM classes can be found here.