Canon Speedlite 600EX-RT Instruction Manual page 71

Remote Release from a Slave Unit
Connect the release cable while the power of the camera and the
Speedlite is off.
Shooting is not possible when focusing with autofocus fails. Focusing
manually before performing remote release is recommended.
The "Release Cable SR-N3" (sold separately) is for an N3 type remote
control terminal. It cannot be used with cameras equipped with a remote
control terminal other than the N3 type.
Remote release is performed with "Single shooting" regardless of the
camera's drive mode setting.
When there are two or more master units, remote release is performed
using the master unit with the <D> lamp lit in green.
69