Preconditions prevent the requested method from applying to a resource other than the one intended. An example of a precondition ...

Preconditions prevent the requested method from applying to a resource other than the one intended. An example of a precondition is to test for expired content in the page cache of the client. When the If-Match or If-Unmodified-Since header is part of the request to the Web server, IIS verifies that the request can continue processing based on these header values. In the case of the If-Match header, IIS checks its value against the current list of E-Tags it has stored. In the case of the If-Unmodified-Since header IIS checks the value against the last modified time of the file.