When it arrived I had a look at it and actually read the manual for a change. I shows how to install batteries and turn it on and beyond that doesn’t even mention the slave mode so I went back to my usual mode of ignoring the manual and working it out myself. I got it only for the slave function and didn’t try it on the camera. It charges up reasonably quickly and with the first few attempts didn’t respond to a 1/128th power flash from my SB800. They don’t say where the sensor is - it is most likely in the front because when i did work that is where I aimed the test flash. Eventually I raised a dispute on Aliexpress [I have always received a full refund with disputes which is why I deal with Aliexpress] I did a video of the tests I did and counted the fires vs misfires. I did quite a few tests first to make sure it wasn’t an S2 mode which ignores the pre-flash of TTL flash mode and then fires on the second flash. Not so, it is just totally irregular. It fired twice, missed 4 times, fired 5 times, missed 3 times, fired twice, missed once, fired 3 times, missed 4 times - and near the end of the test I was giving it a few extra seconds to make sure it was charging up [even though the ready light was on for each test shot I did].
Conclusion: The Godox CF-18 was a really good price for a slave flash, perhaps something to put behind the subject as a rim light when your main light fires. I may have got a bad sample but regardless of how much something cost if it doesn’t actually do the job it is absurdly expensive. Maybe others will have a better experience?