Routing preferences in terms of first position to last are often called routing guide waterfall. Routing guide orders are often chosen for carriers that can meet the service requirements of the load and then by price. Usually, a shipper will use service requirements as a filtering mechanism to include carriers that can perform on a lane before considering price. But once they have made the initial selection, carrier order is often dictated by lowest price carrier on that lane.
Source: Understanding the Waterfall Theory of Freight and how it impacts spot trucking rates
