The breadth of the definition is the point of it. By sweeping in notices, circulars, advertisements and letters, section 2(a)(10) makes every communication that offers a security subject to the Act's disclosure and liability rules, rather than only the document an issuer chooses to label a prospectus. The two carve-outs then create the space for ordinary marketing to exist. One covers a communication sent after the registration statement is effective, where it is proved that a full section 10 prospectus was sent to the same person at or before the same time. The other covers a notice or advertisement that states where a full prospectus can be obtained and does no more than identify the security, state its price, and say who will execute orders, together with anything else the SEC permits by rule. The structure is that a short piece of marketing is allowed precisely because the long document stands behind it.
For funds there are two documents, and the short one is not a summary someone wrote. The SEC describes the pair plainly: the statutory prospectus is the traditional long-form document, and the summary prospectus, used by many funds, "is just a few pages long and contains key information about a fund." The important structural fact is that the same key information is required at the beginning of the statutory prospectus, in a standardized order that the SEC sets: (1) investment objectives and goals; (2) the fee table; (3) investments, risks and performance; (4) management, meaning the investment advisers and portfolio managers; (5) purchase and sale of fund shares; (6) tax information; and (7) financial intermediary compensation. Because the sequence is fixed, two funds' documents can be compared item by item without hunting.
The fee section carries a computation the fund does not get to design. Form N-1A requires the fee table to be followed by an Example whose stated purpose is "to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds." The assumptions are prescribed: a $10,000 investment held for the periods shown, redeemed in full at the end, with a 5% return each year and operating expenses unchanged. The result is a set of dollar figures for one, three, five and ten years. What the fees themselves mean, and how they compound, belongs to the expense ratio and the sales load; the point here is that the comparison is standardized by rule rather than assembled by the reader.
What a prospectus is not. It is not a recommendation, and it is not a prediction. It is also not the complete file: a fund's prospectus points to a longer statement of additional information for the material that does not belong in a document meant to be read. Municipal securities are a separate regime again, offered under a disclosure document of their own rather than under a registered prospectus.