Well from what i’ve read of the nip, it can be a list of pubkeys, hashtags, or events that a user follows
Discussion
You could use kind 30001 (nip 51) "Note Event List" to inform your client that you want to follow all replies to specific events, or you want to *avoid* all replies to specific events. Then if all porn was in reply to a porn topic event, you could avoid that. Is that the idea? It is different from the other three, though I think using a topic tag would be more direct.