I created a data model that contains an interface (this is to allow polymorphism). When a type references the interface then the the Data Management table shows this error:

It seems the UI relies on every reference to contain an externalId field, and that this field is implicitly defined for types but not for interfaces, as the GraphQL query returns with this error: "Validation error of type FieldUndefined: Field 'externalId' in type 'Twin' is undefined @ 'listEquipment/items/twins/items/externalId'"

In the above Twin is an interface type, and Equipment contains a field which is a reference to an array of Twin.