roSGScreenEvents are events sent to a scene graph roSGScreen by the framework.  Other than when notifying the channel's main BrightScript thread that the screen is being closed, and thus that the channel should be terminated, channels do not generally handle these events.

isScreenClosed() as Boolean

Sent to the screen when it is closed.  Channels respond to this event by exiting the main BrightScript thread to exit the application.

Handling the isScreenClosed Event to Terminate a Channel in source/main.brs
sub Main()
end sub

sub showChannelSGScreen()
    screen = CreateObject("roSGScreen")
    m.port = CreateObject("roMessagePort")
    m.scene = screen.CreateScene("SimpleCaptionsScene")
        msg = wait(0, m.port)
	    msgType = type(msg)
        if msgType = "roSGScreenEvent"
            if msg.isScreenClosed() then return
        end if
    end while
end sub