304 Not Modified
Meaning
Indicates that the resource has not been modified since the version specified by the client's conditional request headers (e.g., If-Modified-Since or If-None-Match).
When to Use
When the client has made a conditional GET request and the resource has not changed, so the server returns no body to save bandwidth.
Example
Client requests an image with If-None-Match header; if the image is unchanged, server responds with 304 Not Modified and no body, so the client can use its cached copy.
When Not to Use
Do not use if the resource has changed (instead return the new resource with 200) or if no conditional headers were provided (just return the resource normally).
Source
RFC 9110