The Sec-Fetch-Destfetch metadata request header indicates the request's destination. That is the initiator of the original fetch request, which is where (and how) the fetched data will be used.
This allows servers determine whether to service a request based on whether it is appropriate for how it is expected to be used. For example, a request with an audio destination should request audio data, not some other type of resource (for example, a document that includes sensitive user information).
Servers should ignore this header if it contains any other value.
Directives
Note: These directives correspond to the values returned by Request.destination.
audio
The destination is audio data. This might originate from an HTML <audio> tag.
audioworklet
The destination is data being fetched for use by an audio worklet. This might originate from a call to audioWorklet.addModule().
document
The destination is a document (HTML or XML), and the request is the result of a user-initiated top-level navigation (e.g. resulting from a user clicking a link).
embed
The destination is embedded content. This might originate from an HTML <embed> tag.
A cross-site request generated by an <img> element would result in a request with the following HTTP request headers (note that the destination is image):