it's the specialty boards designation.
F.A.C.S means that a doctor is a Fellow of the American College of Surgeons.
this means that person is "board certified". It is not a requirement to become board certified. But if you want to practice in many hospitals or an academic setting it usually is required. Sometimes you are expected to be "board eligible" before a hospital will grant a physician privileges.
Most surgical specialties have a written and oral examination which are conducted at different times and often different locations. Other specialties have just a written exam.