The particular person, thing, or matter in question; the relevant or aforementioned (often formal, used in documents/legal/official contexts).
当該の書類を窓口まで提出してください。
Please submit the relevant documents to the counter.
当該項目については別途説明いたします。
I will explain the relevant item separately.
当該者はすでに関係機関に連絡済みです。
The person in question has already been contacted by the relevant authorities.