A diagram helps you represent the architecture of your systems and the different dataflows between them.
A diagram item represents a system or anything that interacts in the environment.
A dataflow represents the path taken by messages or any other type of information within the environment. It also represents the configuration needs by systems to communicate this information.
A dataflow segment is a part of a dataflow. It represents a link between two items (systems).
A dataflow segment item is one end of a dataflow segment. It represents one of the two end point between two items (systems).