flatpress/fp-includes/smarty-4.3.0/docs/programmers/api-variables/variable-cache-id.md

$cache_id

Persistent cache_id identifier. As an alternative to passing the same $cache_id to each and every function call, you can set this $cache_id and it will be used implicitly thereafter.

With a $cache_id you can have multiple cache files for a single call to display() or fetch() depending for example from different content of the same template. See the caching section for more information.