As best I can tell, this diode allows TR3 to power DC BUS 2 if TR2 fails. Additionally, it allows TR3 to power DC BUS 1 if TR1 fails, but only when the DC cross bus tie relay (R9 controlled by SPCU) is closed, which is most of the time. It normally only opens at AFDS G/S capture (also opens if bus transfer switch is not in AUTO or either or both AC transfer busses not powered).