The NIPS protocol has introduced a new feature called "reaction events" which allows users to react to other notes. This feature includes the `e` and `p` tags from the note being reacted to, which enables users to receive notifications of reactions to posts they were mentioned in. The last `e` tag must be the ID of the note being reacted to, and the last `p` tag must be the pubkey of the event being reacted to. Additionally, if the client specifies a custom emoji `:shortcode:` in the reaction content, it should render the content as an emoji using the specified URL for the image. Only one `:shortcode:` is allowed per content, and the emoji tag should be one.

Link: https://github.com/nostr-protocol/nips/blob/master/25.md