Currently Esri Maps for SharePoint requires you to log in to one particular portal that has been configured for the SharePoint server. This is either ArcGIS Online or Portal for ArcGIS. This is set up at install time.
If we have multiple portals set up across our organisation but only one SharePoint server, we would like users across our business to be able to log into Esri Maps for SharePoint in SharePoint using their existing Portal Named user account. We don't want to have to set up duplicate named user accounts in multiple portals in order for users to be able to log into Esri Maps for sharepoint. A users ArcGIS identity should work across the whole platform.