Step with me if you will onto the slippery slope of enterprise architecture definition. I try to capture the boundaries of the enterprise in a single simple layered boxes-and-lines diagram. Relationships are conveyed through adjacency and separation helps define inappropriate lines of communications. While the definition is my preferred first deliverable it is based on at least three categories of gathered information and models.
Developed incrementally, using each to validate the others. Each and every change cascades back onto the other models. There will be the temptation to leave partial sketches behind and start over. RESIST !! A few complete, cohesive, balanced visualizations are all you need for the first pass.