I have started to research the purchase of an external flash for my600D. I'll try to lay out what I think I understand and want, and hopefully that will make it easier for you to correct my mistakes and advise what will suit me.
Firstly, my 600D supports wireless flash, so, as I understand it, I could use the built-in flash as the commander/master and buy a cheaper slave unit like the Yongnuo YN560III. I am slightly confused about E-TTL though. The inbuilt flash uses E-TTL to control , right? What happens when using it as a master to control a wireless slave? Does it still achieve E-TTL control, or does that depend on what I buy as a slave? Are some slaves manual only (the YN560 III above?) and others support E-TTL as remote wireless slaves?
What happens when the external flash is mounted on the camera? For a manual flash like the YN560 II, I suspect no E-TTL can happen, but what about other things like wireless flash control. Again, I suspect only some external flashes can be wireless masters. Are these features that I only get in I buy the YN-565EX and YN568EX models?
Another thing I am considering is HSS - high speed sync. Seems to be only available in the YN568EX model. (I am considering other brands like Voeloon too, just using Yongnuo for examples). Can slave flashes do HSS if the master flash on the camera supports HSS itself?
I am thinking HSS might be useful for me when using flash for macro shots with high ambient , so I can use a higher to movement, but also use the flash at the same time. Is that how it would work?
Lastly, I see some models have RF receivers. If I use RF to control flashes, does that do E-TTL, HSS etc?