MPID

MPIDs are usually used to identify FINRA member firms, typically broker-dealers, ATSs, and market makers. Some firms have multiple MPIDs if they operate different trading systems or have distinct business units.

On some venues or trading systems, a MPID may also be called a FirmID.

MPIDs can be found in market data feeds from some U.S. trading venues, like Nasdaq's TotalView or NYSE's Integrated feeds. Usually, MPIDs can only be found on L3 feeds. For example, on Nasdaq TotalView, MPIDs are attached as a field in the Add Order with MPID Attribution message type.

MPIDs can be used to track specific firms' behavior for modeling and signal construction. However, there are limitations to this: On some venues like Nasdaq and NYSE, MPID attribution is not required and purely voluntary, so firms may leave their orders unattributed to avoid signaling risk. It's also possible for firms to route their orders to different executing brokers to anonymize their flow.

On feeds where MPID attribution is voluntary, firms opt in to display their MPIDs with their orders mainly as a way to signal to other participants that there's low adverse selection in their order flow. This practice is more common among broker-dealers that are executing on behalf of clients, where fill rate may take precedence over positive expectancy on their trades.

MPID directories are available from several sources:

• FINRA Market Participant List
• DTCC MPID directory
• Exchange MPID lists, e.g., Nasdaq MPID list