Example: You see a bid for 50k shares on arca and it refreshes lower equivalent to the prints, but then has 17.5k left and disappears without it printing. Does that mean that the remaining order for 17.5k was cancelled?
When ARCA was at 50k did he decrement or was he taken? (did you see a print for 50k on the bid?) Eitherway... It means he pulled the bid... others (NYSE, EDGX ect..) could remain on the bid.. This is common when reading the tape.
I think if the bid stepped down to bid was taken/withdrawn, if bid stepped up he did not get to buy the shares..
Unless you see 17.kshs in the prints from ARCA then you have to conclude he pulled his bid. Now he could either get taken in one print or dozens of smaller prints which would then cause him to decrement. The bids could stepdown but only if ARCA was the only one on the bid, but if NYSE, EDGX, BATS or anyone else was at the same price on the bid, then the bids shouldn't stepdown unless they too pull their bid or are taken.. Unless you see 17.5kshs in the prints from ARCA you can only give an educational guess he pulled his bid, judging from the OP it sounds like there wasn't a print for 50k or 17.5K my guess is he pulled his bid. A little tape reading beginners trade ----------- If say he was taken for the 50kshs on ARCA on the bid (you see a print for 50k on the bid), then refreshed to 17.5kshs, you would automatically want to pay the offer or step in front of the bid to get long (if possible), because judging from the tape he really wants the stock, he took 50k, now wants more, the offers would see this and step up to sell it to him higher (because this guy on the bid wants more, why should they sell it to him cheaper and not see if he will pay for it higher). The high probability play to to get long and get out if the 17.5kshs are taken THEN the bids stepdown because his order is now finished. Ideally we would see the 50kshs taken with 1 print, then 17.5kshs taken or decremented, then a held bid at the same price (ARCA showing 100shs, but prints on the bid for a lot more then he's showing), we would want to get long and stay long until that held bid drops. Held bids or offers are a great way to make a cash flow scalp trade, low risk, your out is if that held bid/offer drops or lifts.