List handling used to be called Iteration strategy in older versions of Taverna. The mechanism is still the same, except in Taverna 1 the implicit and explicit cross product were different. This could cause some confusion between workflow runs as the port order of implicit cross products were not conserved in Taverna 1.
Changing iteration order of cross product
Running the workflow again with the new list handling should reveal 3 lists
[a1, b1, c1, d1],
[a2, b2, c2, d2],
[a3, b3, c3, d3].
Due to a bug in Taverna 1 the order of ports in the Cross product was reversed every time a workflow was saved and opened. When Taverna 2 opens such Taverna 1 workflows, it will ensure the port order of the cross product is the same as if you opened the workflow in Taverna 1 and ran it immediately. Once you save this workflow from Taverna 2, the port order will be preserved correctly.
Pairing up values with dot product