a message flow is only needed when two entities are separate apart with other entities in between. If two activities are next to each other and they are internal activities.
Data object does not necessarily attach to an information flow. It can be attached to process flow if two entities are next to each other.
We don’t need anything in the external pool (vendor) and treat it has a black box. Any message flow sent to that black box points to its edge.
Documents are always attached to arrows, not entity.