In the context of the Navy, PF most commonly stands for "Patrol Frigate."
Patrol frigates were a type of warship designed for anti-submarine warfare and coastal defense. They were smaller and less heavily armed than destroyers but were still capable of engaging in combat. The US Navy employed patrol frigates extensively during World War II and the Cold War.
The term "PF" was used to designate these vessels in the US Navy's hull classification system. For example, a ship with the designation "PF-1" would be a patrol frigate.
However, the term "patrol frigate" has fallen out of use in modern navies. Modern warships that fulfill similar roles are typically classified as frigates or corvettes.
