I believe this is addressed in the manual... but here we go for old times sake (or some sake)...
- This is the only mode where you can map controls to master controls such as the master fade, master speed and the layer set buttons.
- When mapping a control to a layer the parameter you have mapped will control the layer that is in focus, meaning the layer that is currently selected (or highlighted) on the layer stack.
Example: If you map a slider to the playback speed of a layer and layer 1 is selected the slider will change speed the speed of the clip playing in layer 1 and if you change 'focus' to layer 2 the slider will change the speed of the clip in layer 2.
Continuing with the layer speed example, if you map a slider to the speed of a layers playback in this mode then the slider will only be able to control the speed of layer 1. The 'position' means the position of the layer. If you switch layers around, say by switching layer 1 with layer 2, the slider will still only control the layer in that position (position 1).
If you change layer sets the fader will control the speed of layer 1 in position 1 in every layer set.
Position + Layerset
In this mode, if we mapped the slider to the speed of the layer in position 1 then we would only be able to do this in the layer set we mapped the slider to. So if we mapped the slider to the speed of layer position 1 in layer set 1 and then switched to layer set 2 then the slider would not change the speed of the layer.
In general one should take great care when doing midi mappings. One of the biggest caveats is accidentally binding the same control in two different modes. It can happen if you are working on something complex. I usually prefer to stick with one mode when it comes to assigning specific controls for layers.
Keep things simple, rehearse and you should not have any troubles.