Remote triggers: TTL vs ETTL ?
Hi, The term "TTL" may vary slightly between the brands, but generally it was the current flash metering tech at the height of film days at 35mm size. Essentially the metering was done in real time during the actual flash exposure. With digital chips, "off the film" (during exposure) metering became impractical, so manufacturers developed the 'pre-flash' based flash metering systems, eg ITTL & ETTL (now II).
These offer both metered automatic flash exposure across a compatible multi-unit network, as well as Manual Flash (that's Manual but remotely set:adjusted from the commander:master:controller device). FPSync aka HSS is available under both.
Radiopopper PXs piggyback their RF tech:signalling:range onto the top of CLS or Canon Wireless - whereas - the TTL PW Mini & Flex rework:re-engineer the CLS or Canon Wireless processes. This is both good & bad - the interference thing has been addressed, but there are still continuing reports of the PW TTL system rendering 580(II)s into 'pre-flash' only devices (i.e 'capture-flash is left malfunctioning). Conversely, you hear from happy Mini & Flex users too.