When submitting events to relays, clients currently have no way to know
if an event was successfully committed to the database. This NIP
introduces the concept of command results which are like NOTICE's except
provide more information about if an event was accepted or rejected.
A command result is a JSON object with the following structure that is
returned when an event is successfully saved to the database or
rejected:
["OK", <event_id>, <true|false>, <message>]
nip20: https://github.com/nostr-protocol/nips/pull/62
