This document defines the Semantic-ETag HTTP response header field and two associated conditional request header fields. Unlike the standard ETag field, which identifies a selected representation, Semantic-ETag identifies a server-defined semantic equivalence class for the underlying resource state. This enables origin servers to support representation-independent validation and optimistic concurrency control across different representations, such as HTML, JSON, and Markdown views of the same logical resource. This document does not update or replace the semantics of the ETag header field defined in RFC 9110.