under the tax laws, brokers are required to report tax basis, when available, for all sale/closing transactions for securities acquired after 2013. proceeds must be reported regardless of date. in some cases the broker doesn't have the tax basis or is not required to report it - so those are of the type you're asking about.
a few examples: inherited securities, securities transferred from one brokerage to another, certain investments like publicly traded partnerships where the broker gets no updates as to changes in tax basis.
