If you raise this value, the torque reduction during shifting will be higher. So you lose some momentum during shifting. (but just during the 100ms that your shift (shift reduction) will take, so it does not have a very big effect).
Sometimes when the shift times are really short and the clutch cannot fully handle the amount of power that the engine is running, raising this value will help getting a more direct shift and less slip.
In the ideal world you can lower this value, so the moment reduction will be lower, engine power during shifts will remain higher. However the clutch can have more clutch slip when lowering this value.

Play with the number (don’t go below 100Nm, because it will slip too much in most cases)

Ps: In launch control this value will be close to 0Nm. So that’s why the launch control has no ‘dsg farts’ , there is almost no reduction during launch control.
If you want moment reduction (and ‘dsg farts’) during launch control acceleration, you can adjust (to 100%):
“Reduktion bei Schaltart 'maximale Beschleunigung' und LaunchControl”
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All tuned ECU files that I saw raised the map value "09944 Maximale Momentenreduktion in D" to the top

This map does not do anything unless you set it to a very low value.
The reason it is adjusted in so called tuning, is because people copy paste and have no idea what they are doing.

If you set it lower than what the ECU needs in some cases, then the shifts will become very slow, as it can not request enough torque reduction during synchronization.
But torque reduction amount during synchronization is controlled by a target time on DQ250. TCU does it in closed loop.

The standard value is high enough that it will never interfer - tune or no tune. And I can not think of any reason why you would ever limit it. It's probably a failsafe value of some sort, as it never interfers with the gearbox operation in normal circumstances.