If a market maker wanted to avoid filling option orders from a specific account, what information could they use?

External question — Quantitative Finance Stack Exchange Author: opq Original post: https://quant.stackexchange.com/questions/74819 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. FIX has a few fields that do or could contain identifying information for equity option orders, the “ Actionable Identifier ” field in particular, although the OCC states here that “It will not be visible to the contra party to the trade or any other party.“ Is that true that this field is hidden except during the clearing process? Or can market makers with a pfof arrangement with a broker see a customer’s identifiable information before it is routed to an exchange? There are additional fields in the CBOE FIX spec that could contain identifying info, such as ClientID, CustomGroupID, or AdditionalCientInfo. Are fields like these exposed in the order book?

External answer — Quantitative Finance Stack Exchange Author: pyCthon Original post: https://quant.stackexchange.com/a/74833 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. As you noted, when a market market provides liquidity on a particular exchange or executes orders for a retail broker they would not receive any client information. These fields are relevant when a market market executes an order on behalf of a particular client (agency trading) e.x. a large asset management firm chooses to have it's option orders executed by a market maker to save trading costs. This ensures the proper tracking of orders.