203 Non-Authoritative Information
Meaning
The request was successful but the enclosed payload has been modified from that of the origin server's 200 OK response by a transforming proxy.
When to Use
When a proxy or intermediary modifies the origin's response (e.g., removing or changing headers or content) before relaying it to the client.
Example
A caching proxy returns 203 Non-Authoritative Information to indicate it is returning a cached copy that has been modified from the original.
When Not to Use
Do not use if the response is identical to the origin's content (in that case use 200) or if no transformation was applied.
Source
RFC 9110