Usually you create a Data Flow Diagram that models an entire system, not just a port number on a server.
What do they not understand about a port number on a server? It's like asking for a Data Flow Diagram for Google. You make a request for a resource and get a response. It just so happens that ESRI decided to use port 6080 for their ArcGIS Server, probably because it doesn't conflict with any other known ports of other products that might contain an embedded web server.
Maybe they want you to diagram your GIS infrastructure for them? SDE (data sources), ArcGIS Server, Web Adaptor, and custom web applications, ... how they all relate?