At the moment, the amount of descriptive metadata that is shared with AtoM following a DIP upload is rather limited. Right now, uploading a DIP from Archivematica to AtoM will create a target description, and append all digital objects in the DIP as stub child records to that target description. If there is DC XML that has been added to the transfer correctly so it ends up in the METS of the resulting AIP that Archivematica makes, then it will be written to the parent description that is created - there is a mapping as to what fields are supported here:
You can use Archivematica's Appraisal tab to do some arrangement first, potentially defining different levels of description, adding intermediary levels, etc. See:
Otherwise, most of the metadata captured by Archivematica is technical, administrative, and preservation metadata. Some of this technical metadata will be sent to AtoM as well, about the various digital objects included in the DIP - and even information about the original objects from which the DIP objects were derived. These will be displayed in AtoM's existing Digital object metadata area on upload. See:
AtoM administrators can control the visibility of digital object technical metadata to public users via the Visible elements module. See:
As to why EAD and DC for export options?
For XML, EAD is the most comprehensive XML-based metadata exchange standard for archival descriptive metadata, so it's the option that will best capture all your descriptive data on export. DC support is added because EAD, while widely used in the archival profession, is rarely used outside of it - so if users are intending to send / use data in other GLAM institution portals / sites / catalogs / tools (such as a library special collection portal, perhaps), these third-party tools are more likely to support Dublin Core than EAD.
AtoM also supports MODS 3.5 XML, another descriptive XML-based metadata exchange standard used more commonly in libraries and related institutions than archives - here is an example MODS record from the demo site:
It is similar to the DC XML, in that it does not include mappings for all available AtoM description fields, and will only export 1 record at a time (using links to point to parent or child records, but not including them in the export), unlike EAD, which can include an entire archival hierarchy.
MODS and DC were both originally added early on in the project's history, as part of an early attempt to build a sort of distribution of ICA-AToM more useful to llbraries with special collections. IT was the result of a partnership with Canadiana and some other institutions, and was pitched as the "Digital Collections Builder" - but really it was just AtoM with a different theme and some new features added. You can read about the project's history on our wiki, here:
Hope this helps!