Multisite networks

Activate it on one site or across the whole network. Each site keeps its own settings and its own cached pages: a page is only ever served on the site it was built for, each site's Rules and page lifetime apply to that site alone, and Clear the whole cache on one site leaves every other site's pages where they are. A site where the plugin is not active is simply not cached.

The drop-in and WP_CACHE are network-wide by nature: there is one wp-config.php and one wp-content directory. They are installed once, for every site, which is why only a network administrator sees Set up caching and Set up caching again. For the same reason, deactivating the plugin on one site takes only that site out of the cache and clears its pages. The drop-in and WP_CACHE stay for as long as any other site has the plugin active, even a site that has not started caching yet. They go when the last of those sites deactivates it, or when a network administrator deactivates it for the whole network.