From the manual:
A group of incremental snapshots can be combined into one snapshot byusing the zfs send -I option. For example:
# zfs send -I pool/fs@snapA pool/fs@snapD > /snaps/fs@all-I
How does zfs know that @snapB and @snapC are in between @snapA and @snapD? Is there are parent/child relationship between snapshots? If yes, how can I query the parent of a snapshot?