https://www.mongodb.com/docs/cluster-to-cluster-sync/current/reference/collection-level-filtering/#filtering-with--out 

This section seems a bit lengthy and can be misleading. What I believe it tries to convey is that when filtered replication is enabled,  mongosync won't replicate specific collections that are created with $out if you include them in the filter specification.

And the section title of "Filtering with $out" suggests a use case rather than a limitation. Perhaps "Interaction with $out" would be better.