All of the digital lines on the R Series devices are bidirectional as well as all of the digital lines on the cRIO 9403 DIO Module. As is normal with TTL and CMOS signals, the Digital Input (DI) is high impedance and doesn't require a latch to hold the data and the Digital Output (DO) is low impedance and requires a latch to keep or hold the value written to the output on the output. To make these very different lines use the same connector, a switch known as Digital Enable is used. The figure below shows how this is done.
To keep things simple in LabVIEW FPGA, the DO function writes the desired value to Digital Data (DD) and send a true to Digital Enable (DE) to close the switch. If you want to output on a digital line, just use the DO function. It is also very simple to read back what has been written to a digital line. Just read from the digital line (DI) after writing to it (DO). For more advanced functionality like two-way communication, you will need to use the DD and DE.