此事(cǐ shì) - this matter, this affair in Mandarin

此事 (cǐ shì) is this matter, and it belongs to written Mandarin: notices, contracts, news reports, official letters and the more formal register of a meeting. 此 is the literary word for this, the ancestor of the everyday 这, and it survives in exactly this kind of fixed pairing.

What learners get wrong is register rather than meaning. In speech you would say 这件事 or 这个事, with the measure word doing the work. Dropping 此事 into a conversation with a friend sounds like reading from a form. Hear it, recognise it, then reach for 这件事 when you open your own mouth.

It also sits comfortably at the front of a sentence as the topic, which is why 此事与我无关 and 此事另行通知 read so naturally. Note that 此 never takes a measure word, so 此件事 is wrong; the whole point of the literary form is that it skips the counting step.