Describes whether the request resulted in a cache hit / miss / pass / etc.

Fastly writes this header into requests. It is proprietary to Fastly.

This header is set by our standard boilerplate VCL, generated when services do not upload any custom VCL. It is unused and considered deprecated.

Our current recommended boilerplate VCL for services that define their own custom VCL no longer includes this header logic.

It is a request header, but set on the response phase, and therefore is rarely visible outside of Fastly, but may be included in a backend request if a service performs a restart in vcl_deliver or vcl_error, and then makes another fetch to a backend.