An alternative trading system is legally a broker-dealer. Rule 301(b)(1) requires it to register as a broker-dealer under section 15 of the Securities Exchange Act, and it operates under an exemption from having to register as an exchange. That is the whole architecture in one line, and it explains a great deal: the venue's conduct obligations come from broker-dealer regulation rather than from exchange regulation, and its continued exemption is conditional on complying with the Regulation ATS requirements. Lose the exemption and the business cannot operate in that form.
The legal reason a dark pool can stay dark is a conjunctive test, and it is the most precise thing on this page. Regulation ATS contains an order-display requirement that would push a venue's best prices into the public quotation stream. It bites only where the venue satisfies both of two conditions in the same NMS stock. The first is that the venue "displays subscriber orders to any person (other than alternative trading system employees)". The second is a volume test: during at least 4 of the preceding 6 calendar months it had "an average daily trading volume of 5 percent or more of the aggregate average daily share volume for such NMS stock". Where both are met, the venue must give its highest buy price and lowest sell price in that stock, as displayed to more than one person on the system, to an exchange or an association for inclusion in public quotation data, and must give broker-dealers with access to that venue the ability to trade against those orders on equivalent terms. Read the two conditions together and the consequence is stark: a venue that shows orders to nobody outside itself never satisfies the first condition, so however large it becomes, the display obligation never attaches. Being dark is not a loophole in the rule; it is the condition the rule was written around.
A second provision is effectively the regulation's own portrait of a dark pool. Fair access requirements, which force a venue above a 5 percent volume threshold to write access standards, apply them without unfair discrimination, and keep records of every grant and denial, come with an exemption. They do not apply where the venue "matches customer orders for a security with other customer orders", where "such customers' orders are not displayed to any person, other than employees of the alternative trading system", and where "such orders are executed at a price for such security disseminated by an effective transaction reporting plan, or derived from such prices." That is a description, clause by clause, of the classic dark pool: customer-to-customer matching, nothing displayed, priced off the public market rather than setting a price of its own. The rules never use the vernacular; they simply legislate for the shape.
Which is also why the pool is not a source of prices. A venue that executes at or from a price disseminated by the public reporting system is, by construction, a price taker. The public market does the price discovery and the pool matches at the resulting number, most commonly at the midpoint. So the trade-off an institution accepts is explicit: it gives up the chance to set a price in return for not telegraphing that it wants to trade.
The venue's rulebook is a public document, which is the fact the name works hardest to obscure. An NMS Stock ATS cannot rely on the exemption unless it has filed a Form ATS-N with the SEC and that filing is effective, and the rule makes the form a "report" within the meaning of sections 11A, 17(a), 18(a) and 32(a) of the Securities Exchange Act, which is what puts a false or misleading filing inside the Act's liability and penalty provisions. The SEC publishes each effective Form ATS-N and its amendments, along with any order declaring a filing ineffective, any notice of cessation and any order suspending or revoking a venue's exemption. On top of that, each venue must post on its own website a direct hyperlink to the page where those documents sit. So the operating detail of a dark pool, how orders are ranked, who may subscribe, what order types exist, how conflicts with the operator's own business are handled, is a document a member of the public can read, from a link the venue is obliged to provide.
Why an institution wants this. A large order displayed on a public book is information. Anyone who can see it knows that substantial buying or selling is coming, and can trade ahead of the remainder, so the displayed order moves the price against the person who placed it before it is finished. An undisplayed venue removes the signal. The cost is that the order can only be matched against interest that happens to be on that venue, so a pool that is dark and thin may simply not fill, and the institution has spent time rather than money finding out. Nothing about the arrangement is a secret market: the executions are reported to the public tape like any other off-exchange trade, and the venues also produce their own execution-quality reports, prepared separately from those of the broker-dealer that operates them.
The SEC's own usage is a small lesson in reading sources. In the release adopting amendments to the exchange-member exemption, the phrase "dark pools" appears twice, both times in a single footnote, and the surrounding sentence is instructive. The footnote describes what industry white papers say happens "in dark pools", and then switches vocabulary the moment the agency makes a factual statement of its own, which is that "some ATSs segment orders so that institutional investors do not trade with PTFs", the footnote's own term for proprietary automated trading firms. Descriptive shorthand for other people's writing, and "ATS" for its own claim.
