226 IM USED
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 applied to the current instance.
The actual current instance might not be available except by combining this response with other previous or future responses, as appropriate for the specific instance-manipulation(s). If so, the headers of the resulting instance are the result of combining the headers from the 226 response and the other instances, following the rules in section 13.5.3 of the HTTP/1.1 specification.
The request MUST have included an A-IM header field listing at least one instance-manipulation. The response MUST include an Etag header field giving the entity tag of the current instance.
A response received with a status code of 226 MAY be stored by a cache and used in reply to a subsequent request, subject to the HTTP expiration mechanism and any Cache-Control headers, and to the requirements in section 10.6.
A response received with a status code of 226 MAY be used by a cache, in conjunction with a cache entry for the base instance, to create a cache entry for the current instance.
226 IM USED Source: RFC3229 Section 10.4.1
226 CODE REFERENCES
Rails HTTP Status Symbol
Symfony HTTP Status Constant