These letters have been added by the United States Air Transport Security Administration for aircraft that use the US space on their flights and appear to be limited to security precautions.
These characters appear on suspicious passenger tickets. They can not print their boarding passes at home or travel companies, and they should check the airport section for them.
Those who appear on their tickets should not have too much fear; because they will often be allowed to board the plane, but after checking their baggage carefully, often by hand.
The four SSSS characters also appear on single-way tickets, those who pay for them in cash, or who suffer from contagious diseases that pose a threat to public health.