Fields will be populated after initialisation of receiver.
As soon as the mode is detected, the proper functions for the switches will become available.
The GainSource is used to adjust the gain inflight.
You can set it to a fixed value by first selecting a source to manage the gain, and then select gainFIX. Thiw will set the source to MAX, and the gain to the value it just had.
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There is one mainSwitch, used to set the main Stabiliser modes. Be aware, for the different options of the receiver, the order can be different as you are used.
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There is also one modeSwitch.
The texts clarify which modes can be chosen how.