Button next to mixer gain

I’m using n-track for Android 900706480-arm64.
In the mixer there’s a space to type in a gain setting for a track. What is that button to the left of the gain text? The icon on the button is a dot, but if you click the button it changes to other states that are too small for me to see.

That’s the stereo mode button. It allows to make a mono track stereo (so that effects can process in stereo) or to make a stereo track mono by either averaging left and right channels or by picking only the left or only the right channel.