In case it is helpful, I thought I would briefly explain.
Suppose you have snapshot A on machine 1. You transfer snapshot A to machine 2. You later take snapshot B on machine 1, and you delete snapshot A on machine 1. You want to transfer the incremental from A-B to machine 2. You don’t actually need a copy of the full data in snapshot A. You only need to remember the exact time point of snapshot A. Then, you can just transfer all of the blocks that changed after time point A (up to B). That’s what a bookmark does. It just remembers a specific time point without needing to retain the data content.