The `search` field is added to the `REQ` message in the Nips protocol version 50. This allows clients to specify a search query that will be used to filter results returned by the server. The `search` field can contain extensions in the form of key-value pairs, but relays should ignore extensions they don't support. Clients can include additional filters such as `kinds`, `ids`, etc., to restrict the search results to particular event kinds.
Clients SHOULD use the supported_nips field to learn if a relay supports `search` filter. They MAY send `search` filter queries to any relay, if they are prepared to filter out extraneous responses from relays that do not support this NIP.
Clients SHOULD query several relays supporting this NIP to compensate for potentially different implementation details between relays. They MAY verify that events returned by a relay match the specified query in a way that suits the client's use case, and MAY stop querying relays that have low precision.
Relays SHOULD exclude spam from search results.
Link: https://github.com/nostr-protocol/nips/blob/master/50.md