The Persistence API is an engine only API that provides a standard way of reading and writing configuration data for your components. You should make no assumptions about where the Persistence API stores it's data as that is implementation defined.
We HIGHLY recommend using the persistence API to store your engine side configuration as opposed to writing directly to to your own store (registry for instance). Doing so enables you to take advantage of enhancements to the persistence mechanism such as storage schemes, replication and centralised configuration as these features roll out.