IFS explanation: 

If the amounts of customer order lines, which are in the same consolidated group, sum up to a value which exceeds this amount, then the charge price of connected freight charge lines will be zero and the charge cost will be calculated as usual. The freight free amount applies to customer orders. The Use Price Incl Tax setting defined for the specified freight price list will decide if this is a net amount or a gross amount.

Field Freight Free Field in Customer Order / Order Lines | IFS Community

Hope this info helps.