| PlacementIndexInterface | Remembers which tier holds which object. |
| TierStatusInterface | Asks a store which of its tiers can be reached. |
| DatabasePlacementIndex | The placement index a site runs on. |
| MemoryPlacementIndex | A placement index that lives for one process. |
| Placement | Which tiers hold one object. |
| Tier | One rung of the storage ladder: a bucket, an age band, and whether the copy below it survives. |
| TieredProvider | One store spread across several buckets. |
| TierMap | The storage ladder, nearest first. |
| TierMigrationReport | What one migration pass moved, and what it would not move. |
| TierMigrator | Moves history into the tier its age says it belongs in. |
| TierPlacementRebuilder | Rebuilds the placement index by listing every tier. |
| TierPolicy | Decides which tier an object is written to, read from, and moved into. |
| TierRequirementReport | Which buckets a restore needs, worked out before it starts. |
| TierRequirements | Works out which buckets a restore has to read before it reads any of them. |
| TierTarget | Which bucket one tier writes to. |