Hi Sjaak ,
In our process we r using po order as sales order , and then we r creating outbound deliveries
from po's.
Outbound delivery is a document like a work list for us,and the base of collies.
We r making collies from this list and packing them one by one , we can not add a material which is coming from a different outbound delivery in our existing colli (meaning of colli is a handling unit ) , so to optimize collies we r trying to combine the outbound deliveries ,
I hope it will be clear 
Thanks for attention ,
Regards,
Safiye