I say, and with God's success: The event is: ritual impurity: which is the impurity judged by the legislator, and its impurity is established by the legislator: like the impurity of the one in a state of major ritual impurity (junub) and the one who has minor ritual impurity (muhdith). And filth is: real impurity: which is the essence of impurity in reality, without needing to be established by the legislator: like urine, feces, and so on. Accordingly, the event is a specific term for the legal barrier to performing prayer. And filth is a specific term for the real barrier. The term "najis" encompasses both of them. See: Umdat al-Ri'ayah, 1/156, and God knows best.