Caching Key Cookie
Configuring the caching key cookie is optional.
Evolved Caching is built to be directly compatible with the external caching system Varnish, out of the box. This is made possible by the extension building generic, non user specific HTML cache data for each page, and also optionally generating a cookie, named evolved_key, containing a unique page hash Varnish can use to load and save it's cached content. For information on configuring Evolved Caching to work with Varnish, see the Varnish section of the documentation.
You can enable generation of this cookie under Caching Key Cookie. When enabled the unique page hash cookie will be generated for the next page before the user leaves the current page, meaning it will be present when the new page request hits Varnish. Evolved Caching has already run all of the logic to create the page hash in the cookie, so Varnish can just take the cookie content and use it directly to load or save it's cached content.
You can also optionally include the area in the cookie content. Enabling this option will prepend product, category, cms, home or other to the start of the cookie content followed by an underscore, before the page hash. This has no affect on the generated page hash, but can be included to supply Varnish with more useful data about the page it needs to load should you want to customise your Varnish install.