DlvMode not showing on Create new Return Order Form

When I try to create a new Return Order, the dlvMode field isn't there but it is there when I create a new Sales Order? They use the same form I thought so I can't see the difference, other than the SalesType is different, so is there a setting which stops it showing if the SalesType is a Return Order? If so, where is this set? Because when I try to create a Return Order, it asks me to set a dlvMode which I cant as the field isnt visible