226 IM Used

In the context of delta encodings, the HTTP 226 IM Used status code is set by the server to indicate that it is returning a delta to the GET request that it received.

With delta encoding a server responds to GET requests with differences (called deltas) relative to a given base document (rather than the current document). The client uses the A-IM: HTTP header to indicate which differencing algorithm to use and the If-None-Match: header to hint the server about the last version it got. The server generates a delta, sending it back in an HTTP response with the 226 status code and containing the IM: (with the name of the algorithm used) and Delta-Base: (with the ETag matching the base document associated to the delta) HTTP headers.

IM stands for instance manipulations the term used to describe an algorithm generating a delta.