Concepts

• Snapshot: one recorded world state, numbered in creation order (list shows them oldest first). Snapshots store only fresh rows; reads resolve effective state through fallback, so metadata grows with changes, not with universe size.
• Blob (CAS key): the exact raw chunk payload, hashed with framing included. Identical payloads share one stored copy across all snapshots; rollback restores these bytes verbatim, rewinding volatile tags such as LastUpdate to capture time.
• Diff hash: an opt-in volatile view over decompressed NBT with non-essential tags excluded. It speeds up change detection only and never alters stored blobs.
• Tombstone: an explicit “chunk absent” row. A chunk deleted between backups is recorded as a tombstone, not as silence — this is what lets rollback delete it again faithfully.
• Derived state: region_state fingerprints speed up the next backup. Wiping them costs at most one full ingest, never wrong data.

The authoritative definitions are in ARCHITECTURE.md (“Data & Hashing Model” and “History, State, and GC Model”).