Thanks for the feedback.
From my experience, STBY also is used whenever there is a requirement to change the squawk code in mid flight.
I wonder if most C172s have altitude encoding into the transponder ALT.
Also, if mode "C" is ALT, is mode "A" ON?
What about mode "S"
I believe modes "C", "A" & "S" are not available in most light aircraft, but it would be good to know what they are...