IM Used

The server fulfilled a GET request and the response is a result of instance manipulations.

226 IM Used comes from RFC 3229, 'Delta encoding in HTTP'. Instead of re-sending a whole resource, the server sends a diff against a version the client already has, and 226 marks the body as that diff rather than the full resource. Practically no mainstream software implements it, so treat this one as a spec curiosity.

What 226 looks like on the wire

GET /feed HTTP/1.1
Host: example.com
A-IM: feed
If-None-Match: "a1b2c3"

HTTP/1.1 226 IM Used
IM: feed
ETag: "d4e5f6"
Content-Type: application/atom+xml

About 2xx Success

The request was received, understood, and accepted. This is what you want to see.

Where it's defined

226 IM Used is defined in RFC 3229 §10.4.1. MDN's 226 reference covers browser behavior and examples.