That drawing shows a commutation diode that would be placed across an inductive load to kill the back emf spike.
Look at this darlington:
http://www.datasheetcatalog.org/datasheet/motorola/MJ10012.pdf
It has a built in diode across C and E. This is the body diode, right? Is it there as a byproduct of how a darlington is manufactured (like a MOSFET's is) or is it intentionally made as part of the circuit? I don't think it does anything useful, but I'm not sure.