I can say with 100% conviction CommSec absolutely do use CBOE...

I can say with 100% conviction CommSec absolutely do use CBOE (Chi-x), I have transaction statements from both exchanges. I’ve inquired about how they route their orders and was given the (as you say, pretty unhelpful) “we use whichever exchange will provide the best outcome”…..whatever that means, I suspect it means the best outcome for them, not necessarily their clients. Either way, I’m non the wiser as to how they prioritise and route orders, but I do know for sure they use both exchanges to transact. I’ve had a couple of occasions that I’ve placed a limit order and been lucky enough to get the midpoint, so half a tick lower than my limit buy price.