The secondary must know how long to wait for the latest chunk metadata to replicate to it, so it can refresh successfully.

The command will return the optime of the last chunk metadata write on the primary. The shard primary must always stash the optime of the last chunk metadata write from each refresh, so that it can be returned it if the secondary calls. If the command causes the primary to refresh, it will refresh and return the optime from that chunk metadata write operation. The secondary must send the expectedShardVersion in the command to the primary, in case the primary must go to the config server to refresh.