Couchbase Mobile version 1.3 introduces document expiration, a feature that automates the purging of documents from the local database when they are no longer needed, akin to the existing TTL feature in Couchbase Server. The system stores data as JSON documents and revisions, with deletion and purging offering different methods for managing outdated or irrelevant data. While deleting a document replaces it with a "tombstone" entry and still allows for replication, purging completely removes it locally but not remotely, potentially freeing up space while allowing re-synchronization if the remote document changes. Document expiration simplifies this process by setting an expiration date, after which the document is automatically purged, although the actual purging might experience slight delays. This feature allows for efficient management of local data storage while maintaining the capability to re-access information if needed.