Usage patterns
〈事〉の件でregarding the matter of ~
I'd like to talk a bit about the matter from yesterday.
その件についてabout that matter/case
I'll get back to you about that matter at a later date.
〈数〉件の〈事〉が発生するa certain number of incidents/cases occur
Three accidents occurred this month.
未読が〈数〉件あるto have a certain number of unread messages
I have five unread emails.
Expressions
例の件the matter in question; that issue; the thing we're talking about 掲題の件the subject matter; the matter mentioned in the heading/subject line