Add another layer of validation for the cached objects.

How it works

This behavior instructs the edge servers to compare the request’s ETag header with the header of the cached object. If the headers differ, the edge server sends a new copy of the object. This validation occurs in addition to the default validation of Last-Modified and If-Modified-Since headers.
Validation of entity tags improves efficiency of caches and saves bandwidth by preventing unnecessary data transfer.

This behavior doesn't include any options. Specifying the behavior itself enables it.