Graphical representation of the process of selecting an appropriate unit definition. (A) Determine what is known about the production mechanism of the signalling individual, from the hierarchy of production mechanisms, and their spectrotemporal differences. (B) Determine what is known about the perception abilities of the receiver (vertical axis), and how this may modify the production characteristics of the sound (horizontal axis). (C) Choose a classification method suitable for the modified acoustic characteristics (√ indicates suitable, × indicates unsuitable, ~ indicates neutral).