"In an interaction diagram, is the calling class or the called class at the head of the arrow?"

"The called class is at the head of the arrow. The function on the arrow must be a function of the class at the head of arrow."

This connection summary plot is comparable to the game plans equally picturing any collection of actions. The difference is that the single task within the discussion guide diagram will be described being a body, which could incorporate connection - or even collection images.