Canon Speedlite 600EX-RT Setup page 9

The Canon EXRT 600 Flash unites and the STE3 can be set to
Radio or Infrared.
The Canon EX 580 Flashes and all earlier ETTL models can be triggered
using either the EXRT 600 flash units or the STE3 Transmitter,
however, earlier triggers such as the STE-2 can not fire the radio models.