Speed Required (Dark Pools can have a delay) Cost Time of day(Open/Close should only go to primary-Post and premarket-no smart or dark) If you balance your cost of trading, will that extra action cause you to miss trades? If yes, might be work a default for all trades even if there is a higher cost.
at any given a moment there is a quote for nbbo and arca. assume the quote is the same for both arca and nbbo but with different size showing. why do orders end up in one place vs. another? retail forms who get paid for order flow will automatically send it to NBBO.