not necessary an UML diagram, just a simple way to graphically document the relations between several systems...
the systems can be linked through web services, linked-servers, text files, batch processes, com+ components etc...
I was thinking about a high level map, showing which system is consumming, exposing, resources and by which means...
perhaps a deployment diagram would do, or a completely custom diagram...
what do you use to document this kind of relations?
You can create component dependency diagrams in UML.