The HTTP status code 226 "IM Used" is sent by the server when it processes a GET request that includes an A-IM header, and the response contains one or more Instance Manipulations (IMs).

This status code indicates that the server has applied specific manipulations to the requested resource before returning it. The response can be cached and used in future requests, but its caching behavior depends on the expiration mechanism and Cache-Control HTTP headers.

This allows for more efficient caching and retrieval of resources with applied manipulations.

When you receive the 226 IM Used status code, it means the server is providing you with a modified version of the original resource.

For instance, it might only send the changes or differences between the new version and the one you already have to save bandwidth. The types of modifications applied depend on what you indicated you support in the request.

Keep in mind that the original version of the resource may not be available, so you may need to combine this response with previous or future ones to recreate it, depending on the applied modifications.

In this example, the client requests a PDF file from the server and supports gzip compression. The server responds with the compressed PDF and a 200 OK status code. In a subsequent request, the client checks if the document has changed using Etag and If-None-Match headers, and also indicates willingness to accept a delta file created using the VCDIFF algorithm with gzip encoding.

The server recognizes the change based on the Etag header, generates a delta file, and sends it to the client. The client uses the delta file to update the PDF document, and the Delta-Base header ensures the correct base file is used. The A-IM header specifies the applied instance manipulations. This process reduces bandwidth usage and speeds up data transfer.

When the server responds with the 226 IM Used status code, it means that the client requested a resource with an indication that it supports instance manipulations (IM).

The server has fulfilled the request by providing a modified version of the resource, which could be a delta or a compressed version, saving bandwidth and improving data transfer efficiency.

The client can use the provided information to assemble the updated resource and make use of the modifications without downloading the entire resource again.