What does it mean to cancel a NYSE OPG LIM order after open? Does the NYSE open auction not happen in an instant?

External question — Quantitative Finance Stack Exchange Author: Gabi Original post: https://quant.stackexchange.com/questions/74531 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I've just placed a LIM OPG order for a NYSE-listed stock, waited until 09:31, and seeing it did not fill, I requested to cancel it (via the API) and, to my surprise, it was successfully canceled. But what does it even mean to cancel an auction order after the auction has happened? Does it even make any sense? Does the auction not happen in an instant, but rather it happens over a span of time? Or does this look like unusual behavior which may have more to do with how my broker is passing messages around internally?