Capacity is the field that explains the firm's compensation, and it is the reason the rule asks for it. The rule requires the firm to say whether it acted "as agent for such customer, as agent for some other person, as agent for both such customer and some other person, or as principal for its own account", and, if as principal, "whether it is a market maker in the security". Acting as agent, the firm arranged the trade with someone else and must disclose the remuneration it received from the customer, unless that remuneration is set by written agreement on something other than a transaction basis. Acting as principal, the firm sold the customer something out of its own inventory, and what it earned is embedded in the price rather than shown as a separate charge. The trade can look identical on both sides of that line, and the cost to the customer is arrived at in completely different ways.
The agency disclosures go further than most readers expect. For an agency trade the firm must also disclose the name of the person the security was bought from or sold to, or the fact that the information will be furnished on the customer's written request, and the source and amount of any other remuneration it received in connection with the transaction. For a transaction in an NMS stock the confirmation must state whether payment for order flow was received, and that the source and nature of that compensation will be furnished on written request. That last item is a doorway rather than an answer, and what lies behind it is a subject of its own.
What FINRA adds on top of the SEC's list. FINRA's confirmation rule requires, beyond conformity with the SEC rule, the settlement date for transactions in NMS stocks and securities reported under its 6600 Series, other than direct participation programs; a statement that a callable equity security is callable, together with a line telling the customer they may contact the firm for more information; and, in the paragraph that matters most to a bond buyer, the firm's mark-up or mark-down "expressed as a total dollar amount and as a percentage of the prevailing market price". That last requirement is conditional in two ways: the firm must be acting in a principal capacity in a corporate or agency debt security with a non-institutional customer, and it must have bought or sold the security in one or more offsetting transactions of at least the customer's size on the same trading day. Both conditions matter, and the second is why a bond confirmation sometimes carries the figure and sometimes does not.
The institutional line runs through this document too. Because the mark-up disclosure applies only where the customer is non-institutional, the same classification that governs how a firm's marketing material is supervised also decides whether a customer sees the firm's compensation written on the confirmation. A customer whose account meets the institutional definition is presumed able to work the number out unaided.
The rule's own opening note warns against reading it as a ceiling. The Preliminary Note to Rule 10b-10 states that the requirements "that particular information be disclosed is not determinative of a broker-dealer's obligation under the general antifraud provisions of the federal securities laws to disclose additional information to a customer at the time of the customer's investment decision". In other words, a firm that discloses exactly what the confirmation rule lists has not thereby discharged its obligations. The confirmation is a floor.
What the document is good for, in practice. It is the contemporaneous record of what actually happened, produced by the firm, at the moment of the trade. Checking it takes a minute and answers a small number of questions well: was this the security intended, was the quantity right, was the price what was expected, and in what capacity did the firm act. A discrepancy found now is a correctable error; the same discrepancy found at tax time is an argument.