This whitepaper from the ArcGIS Enterprise product management team, discusses the two ways data can be handled with the ArcGIS Platform:
User Managed: where data owners and admins are responsible for providing and managing the underlying storage infrastructure, e.g., file servers and database servers.
ArcGIS Managed: the concept of hosted data because the data is stored within (hosted by) the ArcGIS system and exposed solely through REST based web services that are shown to end users as layer items within their portal.
Also for ArcGIS Enterprise users, the ArcGIS Data Store is not a replacement for existing enterprise geodatabases. .