SF meaning in Military and Defence ?

Special forces and special operations forces (SOF) are military units trained to conduct special operations. NATO has defined special operations as "military activities conducted by specially designated, organized, trained, and equipped forces, manned with selected personnel, using unconventional tactics, techniques, and modes of employment".

Special forces emerged in the early 20th century, with a significant growth in the field during the Second World War, when "every major army involved in the fighting" created formations devoted to special operations behind enemy lines. Depending on the country, special forces may perform functions including airborne operations, counter-insurgency, counter-terrorism, foreign internal defense, covert ops, direct action, hostage rescue, high-value targets/manhunt, intelligence operations, mobility operations, and unconventional warfare.

In Russian-speaking countries, special forces of any country are typically called spetsnaz, an acronym for "special purpose". In the United States, the term special forces often refers specifically to the U.S. Army's Special Forces, while the term special operations forces (SOF) is used more broadly for these types of units.