ATS (Alternative Trading System)

An Alternative Trading System (ATS) is any organized electronic trading platform that matches buyers and sellers of securities but is not registered as a national securities exchange. ATSs are regulated under SEC Regulation ATS (adopted in 1998), which requires them to register as broker-dealers, file Form ATS with the SEC, and report all transactions to FINRA's Trade Reporting Facility.

Types of ATS

ATSs span a wide range of trading venues: Non-displayed venues are the most commonly discussed type — private trading venues where institutional investors trade large blocks away from the public order book. Orders are invisible before execution and reported post-trade. Electronic Communication Networks (ECNs) are automated systems that display orders and match them electronically; unlike non-displayed venues, ECNs typically publish their order books and provide pre-trade transparency. Crossing networks match large institutional block trades at midpoint prices, minimizing market impact without routing through public exchanges.

ATS vs National Exchange

The core regulatory distinction is that national exchanges (NYSE, Nasdaq, CBOE) perform a price discovery function for the public — they publish displayed order books and are required to report best bid and offer prices to the consolidated tape as they change. ATSs have no obligation to provide pre-trade transparency. They are venues for execution, not venues for price discovery. The implication for traders: prices formed in ATSs are influenced by, but do not directly contribute to, the public NBBO.

ATS Volume in US Equities

These venues carry a significant share of US equity trading volume, and FINRA publishes their volume by venue and security. That volume is only part of off-exchange trading, which also includes trades that broker-dealers internalize outside these venues. The fragmented execution landscape — dozens of ATSs competing with each other and with public exchanges — is why order routing algorithms are so important: the best execution for any given order depends on which venue offers the best price and liquidity at that moment.

Why ATSs Matter Here

The post-trade reporting requirement for ATSs creates the data stream that venue-level monitoring tools aggregate. TradeAI News does not ingest ATS data. What it measures instead is unusual volume: how total traded volume in a ticker compares to its own baseline (RVOL), from public market data — shown in the Unusual Volume module. That venue-level data is sold by dedicated providers, and we are not one of them.