If a nonanimated instance (A) sends a message, it is shown originating from the system border.

If an animated instance (B) sends a message to a nonanimated instance (A), it is shown being sent to the system border.

A message sent from a nonanimated class is shown coming from its call stack predecessor, if present. Messages sent from a nonanimated class whose call stack predecessor is absent are shown originating from the system.
Messages sent from one nonanimated class to another are not shown.