we often see posts about so called after hours AH trades. The bulk of the volume from prints one sees after 16:00 eastern time are in fact NOT truly after hour trades.
since the nyse is the primary exchange for HK, it always "prints" its closing trade where it matches all the buy on close and sell on close orders sometime after 16:00 even though it is technically the last print of the REGULAR session. This can come a few seconds or few minutes after 16:00. Because the minor exchanges/ecns do not want to take the risk of price change , they typically "Layoff" their market on close orders to the NYSE , so following the typically larger NYSE closing print, , you will usually see reprints on those other exchanges which have received their allocation from the NYSE closing print. This results in some duplication of volume. In addition to those 'market on close prints" you often will see other "late' prints of trades that took place during the regular session , but for one reason or the other were not printed in a timely manner. If you have a Bloomberg machine, these prints will be marked under the 'condition' column and marked as late or 'out of sequence prints'.
Typically on HK , the closing nyse print has recently been around 100,000 to 200,000. Today because there was a triple wtiching option expiration and a quarterly rebalancing the print was larger. At 16:02:29 the NYSE closing print was 1.8 million at $4.76. Probably because this was a busy close because of the triple witch and quarterly rebalance, the other exchange/ecns printed their allocation of this nyse close late, (they had a lot of stocks to do this with) So the five prints beginning at 16:17:58 to 16:30:18 at $4.76 were these allocation prints. NONE of those six prints were after hours trades and they totalled over 2 million shares (with some duplication).
Hope this helps.