Another way to think about it: An electrical design should contain a schematic, a parts list, a board layout, and a theory of operation. Do the same in software. Don't just give me the code. Don't give me the code plus a bunch of UML. Write a theory of operation. What are the major components? Why are they the major components? How do they interact? Why do they interact that way? How does the system perform the most common actions? How would a new developer make the most likely changes?