#asknostr: what does a client or relay need to implement to be "nostr-complete"?

I was under the impression that most nips are optional, and it would be a natural selection process that determines what is, and what is not, a nostr-complete application (as in users abandoning one app, client, or relay, because they don't implement the most valued features)

which nips make a client nostr-complete?

and what about clients/relays built for specific functionalities?