当人 (tōnin) means “the person concerned”

More examples

最終的な判断は、当人に任せます。sai shū teki na han dan wa tō nin ni maka se ma su

I will leave the final decision to the person concerned.

Identifies the person concerned in a situation, especially when that person is contrasted with people speaking about or judging the situation.

本人honnin: The actual person involved, rather than someone speaking or acting on that person's behalf.

ある事柄の直接の関係者である人。a ru koto gara no choku setsu no kan kei sha de a ru hito

当人 (tōnin) is formed from 当 (tō, "the relevant matter") and 人 (nin, "person"), and the compound means the person connected with that matter.

当人 (tōnin) often appears in formal explanations about whose decision, ability, or experience directly affects the matter under discussion.