description for how to use the GUI

[andrewufrank]

I have a multi-room audio based on snapserver/snapclient working and I have snapweb and snapdroid (as snapcast) on web browser and android. All works nicely and I appreciate the work.
Is there somewhere a description how the GUI works?
I have on top a square <-> triangle, which can switch. What is the idea?
It says for each device "default" and I see not what to do with this.

I assume there is an idea behind it - is there somewhere a description?

thank you!