If I understand correctly Trade Reporting Facility (TRF) volumes represent off-exchange (=ECN) volumes. Why is it said that TRF volumes can proxy retail trading volumes? I see that retail brokerages send much of their order flow to market makers, who can execute off-exchange and report the trade to the TRF, but it is not like non-retail flows all go to exchanges and retails can't go to exchanges. How can you dissect, if possible, the flow breakdown within TRF volumes, say by classification or ticket size.