Here is a riddle: When is an expansion of the federal government’s ability to withhold information not a Freedom of Information Act (FOIA) exemption? When it doesn't amend the FOIA.
In practical terms the distinction between a new FOIA exemption and what is referred to as a b(3) statute (named after the section of the Federal Code that exempts "information that is prohibited from disclosure by another federal law") is practically meaningless: both are ways Congress lets federal agencies keep information out of the public's hands. However, in practice, the fact FOIA is not actually amended when a new b(3) is created has major implications for the level of scrutiny the provision gets before being written into law.