WMS org, Transact Move Orders, Allocate, how to set grouping rules?

 Hi, In a WMS enabled organisation, the Transact Move Orders screen has an Allocate button which allocates material and creates picking tasks.  What determines which Pick Slip Grouping rules it uses?
If I allocate by running the Move Order Pick Slip report there is a field to specify a grouping rule, but I cannot see how to set it for the Allocate button on the form.

Thanks
Mick