우선 - (first of all) ~ be given first treatment
우대 ~ be given special treatment
우선석 = 우대석 = priority seat. (Seat reserved for people with special need i.e. people who need our first consideration*)
- People with special need may then be specified as: 노약자, senior citizens; 임사부, pregnant ladies (not "사부", mothers); 장애인, the disabled ...
Yeah, 노약자석 (老弱者席) is used more commonly. On buses and subway cars you'll usually be able to see both 노약자석 and 임산부석 (임산부 배려석). Most of the time 노약자석 will have an illustration and small written note saying the seat is priority for the elderly and ill, pregnant, disabled or injured, and persons accompanied with child/ren.