Not a code. Probably just to spite a bidder by giving them an immaterial fill (on which the buyer will pay a full commission) OR to make the remaining order no longer “show” on level 2 if now below the “must Show” threshold for order size at that price.
I believe the former happens and I know the latter happens at least mechanically even if not by intent. There are a couple of motives I can think of for this so I think it is intentional.
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