Each packing slip refers to one shipment. One shipment can contain multiple cartons. Each carton will have a container ID. Each container ID will list all of the items within that carton/containerID.
(0333331233656/0333331233657 = container ID's)
0333331233656
122223 1212122 first item 1st carton 10 5
123252 1252542 second item 1st carton 9 9
0333331233657
232525 1235252 first item 2nd carton 5 5
224714 1212525 second item 2nd carton 2 2
If there is only one carton/container ID it does not print in the group header. If there are multiple cartons/container IDs then they all print just fine in the group header.