You can use the 321 positions to manually downshift the gears- in 3 it will be locked out of 4th gear and only use 1st, 2nd and 3rd. I rarely find a need to manually upshift in an automatic- most of the time you want to kick it down a gear in preparation for overtaking, for example. The TCU will intervene to prevent a downshift it that would mean overspeeding the engine.
There is also the mode switch, which switches to Sport mode in high range, and manual mode in low range. Here the D321 selector will choose the numbered gear if the engine speed allows it (D selecting 4th gear in this case.) So by doing some wiring, you could have a manual mode option in high range if you really wanted to.