If the resource representation at that URL ever changes, a new and different ETag is assigned.
It is one of several mechanisms that HTTP provides for web cache validation, which allows a client to make conditional requests.
This allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed.
The use of ETags in the HTTP header is optional (not mandatory as with some other fields of the HTTP 1.1 header).
The method by which ETags are generated has never been specified in the HTTP specification.
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