pre-noun adjectival
the ... in question; the very ...; the ... itself
Pre-noun adjectival (rentaishi) that specifies the exact person or thing being discussed, often with a nuance of 'the very one' or 'the one at issue'. Common in news and formal contexts.
The person in question knew nothing about it.
Please explain the incident in question in detail.
The very person in question knew nothing at all about the rumor.
The very product in question was already sold out.
当該 is a stiff, formal/legal term meaning 'the relevant ...', whereas 当の is more neutral and usable in everyday writing and speech.
当の is an adnominal form placed before a noun to mean 'the very ... in question', while 当 attaches directly as a prefix (as in 当社, 当日).
the person in question; the very person
Compound of 当 (tō, 'this' or 'the relevant') + の (no, attributive particle). The exact historical derivation is uncertain, but it functions as a fixed rentaishi in modern Japanese.