The <ReplayCache> element configures the plugin used for the caching of message identifiers to prevent replay attacks. The only supplied cache implementation relies on a <StorageService> plugin. If this isn't specified/overridden, a built-in MemoryStorageService is used to track the information in memory. Of course, this makes the cache per-node only.
Reference to the id attribute of a previously defined <StorageService> element, to leverage a non-default storage option.
If you actually have to configure this element it would be in order to layer it on top of an alternate storage option. Assuming a previously defined <StorageService> element with an id set to "MyDatabase", the following would install a replay cache using it: