In cases where a resume token or starting time is specified when opening a change stream, the postBatchResumeToken returned with the first batch must always be at least equal to the specified resume point, even if the batch itself is empty. However, if the user opens a change stream on mongoS with a startAtOperationTime at a point in the future (which is perfectly legal), then the stream will return high-water-mark PBRTs that reflect the current clusterTime rather than waiting until the clusterTime exceeds startAtOperationTime.