I have many attribute groups in Contact Builder and all of them, when I click into them and look at the Data Extension linking, they all show Data Source as Unassigned. It doesn't seem to be affecting anything as everything is still working properly. Does anyone know what this does, if anything? The documentation is not clear at all on this, or Custom Data Sources for that matter.
You can create a custom data source and then assign attributes to the custom data source. This is informational only and is intended to be used for data management purposes.