Durability

Writes aren't synced, except where a document could otherwise be left incomplete; a crash may lose any of them. For tests and bulk loads that can be repeated.

Writes are acknowledged once written and synced every interval; a crash may lose writes acknowledged since the last sync. A document is still either completely present or completely absent after a crash.

Every group of writes is synced to disk before it's acknowledged, so acknowledged writes survive crashes. The default.