Core terms
Market participant identifier (MMID / MPID)
The short code on Level 2 that labels which exchange, market maker or other participant is displaying a quote. FINRA's rules call it a market participant symbol (MPID).
FINRA Rule 6160 uses the term "Market Participant Symbol" (MPID) for the identifier a firm uses when reporting trades to a FINRA facility; traders usually say MMID. On a retail Level 2, an exchange's code sums every unrelated order sent to that exchange at a price, while a market maker's code shows one firm's displayed quote. Orders that traders route to an exchange appear under the exchange's code, not under the trader's name.
An identifier shows who displayed a quote, not why, what else the firm holds or what it does next. A registered market maker can be on the offer because its quoting obligation asks for a quote while its inventory is long, short or flat. Wholesalers, which fill much retail order flow, often show little on the book at all; their trades appear later on the tape as off-exchange prints.
"The ax"
Trader slang for the market maker said to control a stock's direction on a given day. The label is usually assigned after the move, and no published count links a refreshing identifier to later direction.
What traders should verify
Log what you can count: displayed size, refresh speed and where prints land against the quote. Treat the identifier as the least informative field on the line.
Do not conflate
A retail Level 2 package may show only some venues, and identifier codes differ between platforms. Reading intent or position from an identifier is inference, and the same refreshing offer appears before breakouts and before fades.
Primary source
Primary sourceFINRA Rule 6160