being directly concerned; matter in question; concerned party or matter
Refers to direct concern or involvement in the matter at issue. Practical learner usage is mainly in compounds such as 当事者, 'party concerned,' and 当事国, 'country directly concerned.'
Please discuss it calmly between the parties concerned.
That country is not a party directly involved in the conflict.
当該 is a formal modifier meaning 'the relevant/said' that identifies what is being referred to, whereas 当事 specifically implies being directly involved as a party in the matter.
Sino-Japanese compound written with 当, associated with what is relevant or in question, and 事, 'matter' or 'affair.' This is a component explanation of the spelling rather than a precise historical derivation.