Yes. it's 0%, 50%, and 100% of the *programmed* value. Since the max is 80, then it being 40 at 50% makes sense. And it probably never actually closes to zero.
As far as how the throttle works, this is only really the basics, but it gives the idea and these are the most important maps.
This is the *mechanical* limits of the throttle opening:
This is the actual min/max TPS setpoint (note that "0%" is really the mechanical limit):
What does the throttle do other than serve as a backup to valvetronic? Well, it can be used to generate vacuum in certain cases, and help with engine startup. Here's the engine start positions based on coolant temp:
Here's the map that is directly referenced in that INPA function (throttle setpoint external adjustment):
There is some hysteresis involved as well, and this map is specifically from a 330i. I assume the 328i is similar.
Here's the throttle mapping based on pedal input (left axis) and maf flow in kg/hr (top axis). This is what directly affects your pedal input, it's multiplied by the c_tps_max values to get the actual throttle position:
Note the left axis scale. It doesn't even start until 53% pedal input, and most of the resolution is from 90-99%. And the top axis is the same, most of the resolution is at low airflow (think about idle, coasting, low speed parking lots, etc), where at high ariflow where you might be cruising down the road, it's mostly open (I don't recall exactly, but I believe the engine flows a lot more than 360kg/hr).
I didn't dig it up but there's also a pedal input voltage vs throttle movement value somewhere, you could use it to make the "throttle" more sensitive or less, but actually that is more related to Valvetronic and the valve opening values, not the throttle itself in a classical sense.
Of course, there are other functions and maps that will determine the final throttle opening, but it gives you the idea.