The scale mode is only relevant when you switch the layer while the potentiometers current position doesn't match the value of the layer. It's difficult to describe with words, but once you actually try it, it's super simple to understand.
If you prefer to read an explanation, click here for a lot of text
To give an example: Lets assume that the outputs from the three layers (0, 1 and 2) are currently 20, 34 and 56 respectively. The selected layer is 0. Let's also assume, that the potentiometer is currently turned to value 20, matching the value of the selected layer.
What happens when you switch from layer 0 to layer 2? In any case, the value of layer 2 won't change until you physically move the potentiometer. The three modes determine what the value will be set to when you start turning the potentiometer.
In overwrite mode, when you turn the potentiometer for the first time after the layer change, the output of layer 2 will immediately be set to the potentiometers current position. In our example, the output of layer 2 was 56 and the potentiometer was set to value 20 before the layer was switched. Once you touch the potentiometer and turn it to value 21, the output of layer 2 will jump from 56 to 21 instantly.
In pickup mode, when you turn the potentiometer for the first time after the layer change, the output of the layer won't change until the potentiometer is turned to the value of this layer. In our example, you will have to move the potentiometer to the value 56. Once you reach that value, you will be able to modify the value of layer 2. This prevents sudden jumps in the value of layer 2.
In scale mode, the remaining travel of the potentiometer in both directions will be mapped to the remaining travel of the value. That means: If you start turning the potentiometer clockwise, the value of the selected layer will increase so that once the potentiometer reaches its maximum position, the layer will also have reached its maximum value. If you turn anti-clockwise the same applies: The value will decrease, reaching 0 when the pot reaches its minimum position. In our example, the value is 56 and the potentiometers position is 20. That means, when you turn the potentiometer clockwise, moving the potentiometer from 20 to 64 will change the value from 56 to 64. turning the potentiometer anti-clockwise from 20 to 0 will change the value from 56 to 0. That means that the value will decrease much faster than it will increase. But this mdoe has the advantage that it avoids both sudden jumps in the value (as happens with the overwrite mode) and "dead zones" (as happens with pickup mode)