226 IM Used
Meaning
The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations (Delta encoding) applied to the current instance.
When to Use
When a server has applied a delta encoding to a representation and is returning the result (indicating that the response is not a full new resource but a manipulated instance).
Example
A server returns 226 IM Used in response to a GET that included an "A-IM" header (Acceptable Instance-Manipulations) to indicate it applied a delta update to the content.
When Not to Use
Do not use if no instance-manipulation was applied; use 200 OK or another appropriate status for normal responses.
Source
RFC 3229