The 4: 40 PM closing rotation on the SET.

The SET  final closing rotation only applies to the closing session in the afternoon.

Meaning, that if you enter an order after the "closing bell" at 4:30 PM, you can still execute a trade for 10 minutes. At 4:40 PM there is what I call a closing rotation, where buyers and sellers are matched in a grand finale, for a official final price of the day, then so published at 4:40 PM. 

To get this final rotation, one must enter the buy/sell order within a few minutes, right after 4:30 PM. You will only get executed on this, if your limit price was below (if you sell), or above (if you buy), to what ends-up being the final price of that day. It is entirely possible you get executed if you entered your limit price at the same price it closed at 4:40 PM. But in that case you are not entitled, only lucky to get it.

The SET index as well hence gets so re-calculated one more time at 4:40 PM or 4:41 PM.  This is why the final SET index closing level for that day is never the same as first shown 4:30 PM. Again, none of this applies to the morning session.