103 Early Hints
Meaning
Used to return some response headers before the final HTTP response is sent.
When to Use
When the server wants to hint to the client (often a browser) about resources it can preload while the final response is being prepared.
Example
Server sends 103 Early Hints with Link headers (preload hints) before sending a final 200 OK response.
When Not to Use
Do not use if there are no preliminary headers to send or if not using link preload strategies.
Source
RFC 8297