In the Create Container page, lower part (Ventures), in the “Add to Container” column, there are the words Create and Add to Container. What is the function of ‘Create’?

The words depend on the status of the PO Shipment (Venture):

If the status is ‘PO Issued’ then theoretically adding that PO Shipment to the Container will MAKE it into a Venture (Create: Venture)

Or

If the PO Shipment (Venture) is already an ‘At Source’ venture (which is the only other option because Containers can only have PO Issued Shipments or At Source Ventures added to them) then you are simply adding it to the container