As I understand it, reportable transactions, meaning reportable to the IRS by the broker, include most trades taxpayers make but not all.
Some trades not reportable by the broker to the IRS include options trades, some OID amounts, and principal payments on REITs and CMOs. ~~~~~~~~~~~ Almost all brokerage sales must be reported on Form 1099-B, but a detailed listing may not be submitted by the broker to the IRS.
I see a lot of brokerage statements and, although the broker provides the client with the cost as well as the sales value, the broker often does NOT report all of this on a transaction-by-transaction basis to the IRS--and will include a statement to that effect somewhere in the "small print." These are "uncovered" transactions.
A "covered" transaction, however, is a transaction for which the broker MUST report both cost and sales figures. In future years, more and more transacitons will be considered "covered."