The newest addition to the growing collection of concepts are HTTP Warn Codes (which takes the total count of concepts to 30). This new concept currently lists 7 values, all of which are from the respective IANA registry (RFC 7234 currently is the only source for defined/registered HTTP warn codes).

As usual, feedback is greatly appreciated, in particular if you are using HTTP Warn Codes or have seen HTTP Warn Codes being used in ways which are not (yet) covered by the values listed so far.