Got it...I understand why dark pools exist but what is the purpose of regional exchanges such as NYSE ARCA?This will show you where the trading is at.
https://markets.cboe.com/us/equities/market_share/
Tape A = NYSE
Tape B = "Regionals" - pretty much NYSE Mkt, NYSE Arca
Tape C= Nasdaq
All dark pool trades need to report into the TRF - but not all such TRF trades come from a dark pool.