Just for sh%ts and giggles while you wait for Terry to reply, here's my understanding.
It is not clear to me if these are JB4 fuel trims or ECU fuel trims but I suspect ECU.
If they are ECU fuel trims:
There are base maps for fuel and timing that the ECU uses as a starting point based on conditions (load, rpm, temp, pressure, airflow, knock etc). But then it has a system to watch the results live and make deviations from those maps. This allows for the car to be more fuel efficient and responsive... and fortunately for us allows it to adapt the some of the changes we throw at it. The fuel trims are deviations the ECU is making live from the map values to stay closer to target. A fuel trim value of 50 indicates tha the ECU is throwing 34% more fuel than the maps expected into the engine.
If they are JB4 trims (or some combination of JB4+ECU) then it's the piggyback making further live adjustments for the same reasons.
I thought fuel trims maxed out at 50. So, if you are seeing it flatline at 50 then you are likely running leaner than the ECU/JB4 intends because it can no longer get enough fuel for the air.. which is bad. But if you are seeing higher then I have to recheck.
What I have seen in trims is an early but relatively ramp to low or mid 40s at WOT onset then settling down towards the mid 20s. Each gear shift will see another rise and then settling down.
Here is what the N54 site says. Doesn't indicate specifically that 50 is max, though I got that impression somewhere along the way. But it does align with what I've seen in typical maps:
Trim: Fuel trims in bank1. The JB4 has scaled fuel trims for quicker viewing, 25 in JB4 logs = 0%, 50 = +34%, and 0 = -34%. Generally fuel trims will jump up in to the 40s under peak torque and drop down towards 25 at higher RPM.
Trim2: Fuel trims in bank2. These will mirror Trim generally and if there is a deviation may indicate a fuel wire is loose or installed improperly. The JB4 will disable if Trim and Trim2 have more than a 15pt variation.
Values early into WOT of mid 40s is perfectly fine and upper 40s are likely okay too as the ECU has not run out of range.
If you are maxing out it could be overboost, fuel pressure issues, fuel pump limits, injector issues or duty cycle limits. You will likely see ing2-6 start increasing here as the leaner afr will cause knock. Overboosting, maxed fuel trims, limp mode... these are all signs of potentially engine damaging conditions.
