Default client cache time to live. Provides a default expiration interval in case a particular catalog record does not contain a valid
vignette::Expiration value. Or, if a vignette file or material file is accessed directly, rather than by way of a catalog record.
0 or greater. Number of hours until expiration since the reply data was generated. Set to
0 to always expire the reply image immediately, which effectively disables client caching. Set to
-1 to mark as never expire.
default::Expiration if not defined or if empty.