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).
Radio is not equal to radio, even if most of the speedlites work within the same radio range. This is because there are frequencies and - even if the frequencies are identical - protocols. Here are a few systems all working within 2.4GHz (2.4G) not fully or not at all cross-compatible (some offer so-called compatiblity modes):
Canon RT, Godox X (GX), Nissin Air System (NAS), Phottix radio, PiXeL radio, Profoto Air, Shanny RF (SRF), Shanny RT (Canon RT), Triopo G.
So if the list says i.e. Godox speedlite model A supports Godox X and Godox model B supports 2.4G chances are they are not compatible. Same for i.e. Shanny RF and RT systems and many more.
When it comes to radio triggers you'll not only find devices working within the 2.4GHz band but also within the 433MHz band. They are working quite well, but...
Canon RT: 2405 - 2475MHz1
Godox X: 2413 - 2464.5MHz @ 5dbm for ≤50m2
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 while 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 (to be used in setups without 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.
1 Yongnuo YN-E3-RX Manual
2 Pixapro Li-ion350IIC Manual
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