Almost everything else in this book depends on having this picture accurately. It is also the question that separates candidates instantly: someone who has only traded describes clicking a button, and someone who has worked in the industry describes eight systems, three of which can reject the order and one of which is a legal entity that guarantees the trade.

• Your application. A mobile app, a terminal, or an API. Nothing has been validated yet beyond basic field checks.
• The broker's risk management system. This is the first place your order can die, and it is where most rejections happen. It checks available margin, product-type eligibility, whether the scrip is banned in the F&O segment today, whether you have breached a limit. Under SEBI's upfront-margin regime this check is not advisory — the broker must have collected the margin before the order goes out, which is a genuine change from how the market worked before 2020.
• The exchange gateway. The broker sends the order over a leased connection to NSE or BSE. The exchange applies its own checks: price bands, circuit filters, tick size, lot size, order-to-trade ratio.
• The matching engine. Price-time priority. A better price always wins; among equal prices, whoever arrived first wins. This is why latency matters at all — not because speed is inherently valuable, but because time is the second sort key.
• The clearing corporation. NSE Clearing or Indian Clearing Corporation for BSE. The moment your trade matches, the clearing corporation legally interposes itself as buyer to every seller and seller to every buyer. You are no longer exposed to the person on the other side of your trade. This is the single most under-appreciated piece of market infrastructure and it is the reason a retail investor can trade with a stranger and not care who they are.
• The depository. NSDL or CDSL. Shares exist as book entries; settlement moves them between demat accounts on T+1.

An American retail order usually does not go to an exchange. The broker routes it to a wholesaler — a small number of very large market-making firms — which internalises it, filling from its own inventory at or better than the national best bid and offer. In return the wholesaler frequently pays the broker for that order flow.

This arrangement is the source of most American market-structure argument. The case for it: retail orders get price improvement over the displayed quote, execution is instant, and it funds zero-commission brokerage. The case against it: the broker's incentive is to route where it is paid rather than where the customer does best, the improvement is measured against a benchmark the wholesaler helps determine, and it removes retail flow from the public order book — which arguably widens spreads for everyone else.

You do not need a position on this. You do need to be able to state both sides, because it will come up in any US market-structure conversation and having a memorised opinion is worse than having a clear map.

• Market — execute now at whatever price is available. You are buying immediacy and paying for it with the spread and whatever depth you consume. In an illiquid instrument this can be catastrophic, which is why Indian brokers apply protection ranges.
• Limit — execute at this price or better. You may not execute at all. You have swapped price certainty for execution uncertainty, which is a real trade rather than a free improvement.
• Stop / stop-loss — becomes a market or limit order when a trigger price prints. Note that a stop-loss market order in a gapping market executes far below your trigger, and a stop-loss limit order may not execute at all. Both failure modes have ended accounts.
• Immediate-or-cancel, fill-or-kill — execute what you can now, cancel the rest, or execute all of it now or none. Used constantly by algorithms and almost never by humans.
• Iceberg / disclosed quantity — show a small part of a large order. The hidden part loses time priority when it refreshes, which is the cost you pay for concealment.

A strong answer to “walk me through an order” names the eight stages, then adds one thing that shows you have thought about it. The best available addition is novation: “and the moment it matches, the clearing corporation becomes the counterparty to both sides, which is why margin exists and why a default is a liquidity event for the clearing corporation rather than a loss for the person you traded with.”