選手 and 補欠: athletes, substitutes, and vacancy filling
Compare an athlete's run, applause for an injured player, a substitute entering, and an added committee member without merging separate people or events.
選手 (せんしゅ): an athlete in the event
「若い選手が決勝で見事な走りを見せた。」 (“The young athlete ran brilliantly in the final.”) Read the athlete's impressive run as the reported action. It does not establish a win or identify this runner with the injured player in another sentence.
「けがをした選手に観客から拍手が送られた。」 (“The crowd applauded the injured player.”) The applause is directed at an injured athlete. It does not report recovery from the injury or a later return to play.
補欠 (ほけつ): substituting or filling a vacancy
「けがをした選手に代わって補欠が出場した。」 (“A reserve player took the injured player's place in the match.”) This reports the substitute entering in place of the injured player. Preserve that action instead of reducing it to a waiting-list status or joining it to a separate 選手 example.
「委員の補欠を一人選んで名簿に加えた。」 (“We selected one substitute to fill the committee vacancy and added the person to the list.”) The committee example selects one additional member and puts that person on the list. It does not make 補欠 exclusive to sports athletes.
Keep each person's role in its own sentence
選手 names the athlete role; 補欠 focuses on a substitute or someone filling a vacancy. A substitute can play as an athlete, so the roles are not mutually exclusive categories.
The runner, injured athlete, substitute, and committee member belong to separate examples. A shared topic is not enough to turn them into one person's sequence of events.
Must a 補欠 added to fill a committee vacancy be a sports athlete?
Show the answer and why
No. The committee example uses 補欠 outside that role. Read the substitute who actually entered and the added committee member within their separate targets and actions.