c# What are these volume changing values/messages

I found this script to change the System sound volume and it works. But what are these constant volume codes called and where can I find a full list of these codes that do more things.

    [DllImport("user32.dll")]
    static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

//Volume codes, or messages, or whatever they are called
const int VOLUME_MUTE = 0x80000;
const int VOLUME_DOWN = 0x90000;
const int VOLUME_UP = 0xA0000;

SendMessage(this.Handle, WM_APPCOMMAND, IntPtr.Zero, (IntPtr)VOLUME_UP);