Simplify only as much as the subject allows. But complexity, too, is in the eye of the investigator. Look as well from the eyes of the customer and other actors within and outside the system. Diagrams rarely stand alone without explanation. A narrative discussion of the topic, which interweaves words, pictures, diagrams, and graphics is normally required.
Every diagram needs a title setting out clearly the purpose and type of diagram.
Use a key if the lines, arrows, boundaries, etc. are not self-evident. Beginners should very rarely need a key because they will be following accepted convention. Diagramming is an art. The skill you develop will be distinctly your own. Systems maps typically use the Venn diagramming convention shown in Carter, pg.