master override meaning?

[shawaj]

In your device tree overlay file you have:

  __overrides__ {
        alsaname = <&slave_overlay>,"simple-audio-card,name";
        compatible = <&wm8960>,"compatible";
        master = <0>,"=2!3";
    };
};

What does the master = <0>,"=2!3"; override do? And how is it used?