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 ...
Yes, sort of.
우대석, preferential seats and 노약자석, the elderly & weak seats differ in their daily practical usage, based on psychology and local culture.
The use of "약자" (in '노약자'), the weak carries the connotation of 겁쟁이(성격이 약한 사람, people with weak characters) i.e. 'wimps'. This tends to prevent people from making use of such facility, out of pride (self-respect), opting instead to stand.
우대, literally means 'special treatment'. This in itself has its downsides in a "계급 없는 사회"(classless society) ...
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.