all_durable: All transactions with timestamps earlier than the all_durable timestamp are committed. This is the point at which the oplog has no gaps, which are created when we reserve timestamps before executing the associated write. Since this timestamp is used to maintain the oplog visibility point, it is important that all operations up to and including this timestamp are committed and durable on disk. This is so that we can replicate the oplog without any gaps. 

The "and durable on disk" should be removed.

URL: https://docs.devprod.prod.corp.mongodb.com/mongo/src/mongo/db/repl/