Canon's wireless TTL was exclusively based on InfraRed signals until 2012, when 600EX-RT and ST-E3-RT were released. With the RT series Canon finally entered the world of true E-TTL radio triggering.
Radio signaling is the most reliable method to keep up communication between unwired equipment. Unlike optical transmissions (IR, preflash or flash bursts) radio nowadays works flawlessly under any ambient light conditions, goes through walls and other optical barriers and has generally a way farther reach - currently up to 500m, whereas optical transmissions under best conditions barely make it up to 15m (i.e. the Canon ST-E2 has a spec'ed range of 12-15m indoors (thanks to reflections off walls) and only 8-10m outdoors).
IR-RRT (InfraRed Relay Radio Triggering): Neither transmitter nor receiver communicate the E-TTL signals with their host via hot shoe. Instead, the transmitter records IR signals sent by the master flash or internal/external ST-E2, converts them into radio signals, which are sent to the receiver. The receiver translates them back into IR, which is served to the IR sensor of the flash. Thus the master and slaves both will have to feature IR compatibility.
Digital Slave (DS) allows the flash to be optically triggered by the master's main flash burst. The slave will fire at full power or at the on-slave manually defined power level (as far as supported). There is no choice of number of preflashes to be ignored. It's just S or die.
S1/S2 are optical alias digital slave modes firing the slave at full or on-slave defined power as soon as the master fires. S1 is commonly used for sets without preflash, S2 for sets with preflash. Some speedlites offer this slave mode handling even more preflashes and usually start counting at S0 instead of S1.
S0 indicates the old analog EOS models' TTL slave mode. Basically identical to S1 mode, fires on the first burst (no preflash).
That said, ...
• on TTL capable cameras (no preflash) ANY flash can be used as master for flashes capable of S0 or S1/S2 modes (set to S1) and ...
• on A-TTL/E-TTL capable cameras (both use preflash) any A-TTL/E-TTL capable flash can be used as master for DS and S2 capable flashes.
This capability is not further considered in the listings under Master capabilities due to triviality.
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