Object cache groups, and which ones are global

On multisite the object cache prefixes keys with the current blog id, so the same key on two sites does not collide. Some groups must not be prefixed — users exist once across the network — and those have to be declared.

wp_cache_add_global_groups( array( 'my_plugin_licences' ) );

wp_cache_set( 'licence', $data, 'my_plugin_licences' );
// same value on every site in the network

Getting this wrong on a single site has no visible effect, which is why it is usually discovered when a plugin is first installed on a network. The declaration has to happen before any use of the group, so it belongs on init at the earliest priority or in a drop-in. There is also wp_cache_add_non_persistent_groups() for data that should never reach Redis or Memcached.