Yes, it certainly will work... But it's sort of massive overkill to get the top-of-the-line 600EX-RT primarily for it's AF Assist feature! It's a great flash, but the Focus Assist is just one of many things it can do!
If the main thing you need is Focus Assist, an ST-E2 Flash Controller module will do the same thing, and is much smaller and considerably less expensive. (Note: the ST-E3-RT does not have Focus Assist).
If you want a flash with Focus Assist, you really can pick and choose among many of the Canon flashes (as well as a bunch of third party flashes for Canon). Among the Canon flashes, Focus Assist is:
- Near Infrared type (less intrusive), 15 to 30 ft. range on: 600EX-RT, 430EX III-RT, 430EX II, 270EX II & ST-E2
- White light flashes (rapid bursts, more intrusive: 15 to 25 ft. range: 430EX III-RT, 270EX, 90EX.
- White light, continuous LED (serves as video light too). Range 10 to 13 ft.: 320EX .
- N/A: ST-E3-RT.
For other purposes... The RT flashes and ST controller use radio to communicate with each other for multi flash setups. The 600EX-RT and ST-E3-RT can act as masters, while the new 430EX III-RT is slave only. Using radio gives them more range and greater flexibilty of placement.
The 600EX-RT (optionally) and some of the other non-RT flashes and controller use optical (near infrared light) to communicate with each other. The 600EX-RT (and the now discontinued 580EX II, 580EX, 550EX models) and the ST-E2 controller can act as masters. The 430EX II (and discontinued 430EX, 420EX) and 270EX II can be used as slaves only. The optical triggering and control requires line-of-sight setup and arrangements, and has a bit less range than the radio control.
Some of the camera's with built-in flash have optical means of controlling off-camera flashes. I don't think the 1100D is one of them, though.
