The HTTP If-None-Match request header makes a request conditional.
The server returns the requested resource in GET and HEAD methods with a 200 status, only if it doesn't have an ETag matching the ones in the If-None-Match header.
For other methods, the request will be processed only if the eventually existing resource's ETag doesn't match any of the Values listed....ETag doesn't match any of the values listed. When the condition...If-None-Match used with the * value can be used to save a file only...