An alternative trading system brings buyers and sellers together like an exchange but is regulated as a broker-dealer. Most dark-pool venues and many electronic bond and crossing networks operate this way, which is why they can set their own access rules and do not have to display quotes.
Form ATS-N forced meaningful transparency for equity ATSs. Operators must publicly describe order types, matching logic, how affiliated principal flow is handled, and who can see what, and the filings are on sec-edgar for anyone to read.
Volume triggers add obligations: above certain thresholds in a security, fair access and systems capacity requirements apply. Aggregate ATS volume is published weekly by finra, which is the standard source for how much of a stock trades off exchange.
Related: dark-pool, regulation-nms, broker-dealer, sec-edgar, finra