Although a few large older ships are powered by steam turbine the overwhelming majority of vessels are today powered by either slow speed or medium speed diesel engines. Particular attention is paid in modern ships to the fuel preparation equipment, enabling vessels to burn low cost residual Intermediate Fuel Oil (IFO) bunker fuels, (even those of dubious quality) efficiently and without harm to the engines. Some older vessels may also consume Marine Diesel Oil (a fuel of better quality) or a blend of IFO and Diesel, in the auxiliary engines used to power the generators and also in the main engine when the vessel is entering or leaving port or while manoeuvring in confined waters. This is because the response of an older engine design to a change of throttle position is very slow when burning IFO bunkers. As this fact could affect the safety of the ship when an instantaneous response by the engine is required, the fuel will be switched from IFO to MDO when the response will be almost instantaneous. Modern vessels will be fully automated, that is to say, the main engine can be controlled from the navigating bridge. In simplistic terms, ships are employed either as liners or as tramps. What defines a ship as a liner is that it trades according to a schedule, between two designated areas and its cargo is made up of very many different consignments from numerous shippers, generally manufactured or semi-manufactured goods. Freight rates are usually published in a tariff and may vary according to the commodity. The trading area will determine the type of vessel which may be a container ship, a Ro-Ro or a combination of two types; general purpose ships are still used in a very few liner trades. The normal document covering the contract of carriage will be a Bill of Lading.