A rule, established routine, or customary practice—things that have been decided and are expected to be followed.
学校にはいくつかの決まり事があるから守ってください。
There are several rules at school, so please follow them.
会社の決まり事に従って手続きを進めてください。
Please proceed with the procedures according to the company's rules.
家庭では食後に片付けるのが決まり事だ。
In our household, it's a rule to clean up after meals.
決まり事だからといって必ずしも正しいとは限らない。
Just because something is a rule doesn't mean it's necessarily right.