Canon Speedlite ST-E3-RT Mode D'emploi page 65

<k> is displayed.
Set the shutter speed 1 stop slower than the flash sync speed (p.19).
Cannot release from a slave unit.
When an EOS camera which was released up to 2011, has an N3 type
remote control terminal and is compatible with E-TTL II/E-TTL autoflash
is used to perform remote release from a slave unit or when it has been
set as the slave unit during linked shooting, the "Release Cable SR-N3"
(sold separately) is necessary (p.42, 43, 60).
Troubleshooting Guide
63