There are multiple idle schedules programed into the FADEC, and each idle schedule has different minimum parameters. The FADEC will pick between those minimum parameters (burner pressure, N2, and occasionally N1) and select the one that provides the highest thrust.
Speaking for Boeing, NOT Airbus (although AB is probably similar in concept), approach idle is automatically selected when Engine/Cowl Anti-Ice is ON, or flaps in landing (20 or greater, or 25 or greater, depending on the model). On some installations, selection of Continuous ignition will also command approach idle.