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This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.
It just tells the browser and proxies to validate the cache content with the server before using it (this is done by using If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match attributes mentioned above).
HTTP/2 makes some restrictions on specific header fields (see below).
Non-standard header fields were conventionally marked by prefixing the field name with The standard imposes no limits to the size of each header field name or value, or to the number of fields.
It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource.
This section defines the syntax and semantics of all standard HTTP/1.1 header fields.
The permanent registry of header fields and repository of provisional registrations are maintained by the IANA.