The following is based on Van Buren and has not been confirmed by canon sources.
UAF stands for Unamerican Activities Force. They were usually Federal agents who worked to ferret out Communist sympathizers in the American population. UAF armor was advanced light armor with a small stealth bonus (the step up from it was Chinese Hei Gui/"Black Ghost" armor). Its complement was UAF helmet.
The name is based on the real-life "House Unamerican Activities Committee" (HUAC) in the US Congress in the 1950s, created to investigate Communists in government and media.