Hi curious one.
Squawk followed by numbers or the word 'ident' is the request by ATC to the aircraft to transmit a number back to the ground which sends ATC details of identity and altitude which are then displayed on the radar screen. Ident gives a pulsing symbol to highlight an single identity on the radar.. .'Heavy', not solely for overweight US aircraft!! reminds ATC of the Wake Vortex category which may mean more spacing on arrival or departure, and also, to a lesser degree, that the rate of climb may be poor.. .Perhaps I should write a book or film!!. .Regards