The ETOPS rating can also determine what routes an aircraft is allowed to fly, depending on the regulations of its governing agency. U.S. registered aircraft (N-number) are governed by FAA regulations. FAR Part 121, Section 161, paragraph (a) states, as of the date of this writeup:
(a) Except as provided in paragraph (e) of this section, unless approved by the Administrator in accordance with Appendix P of this part and authorized in the certificate holder's operations specifications, no certificate holder may operate a turbine-engine-powered airplane over a route that contains a point—
(1) Farther than a flying time from an Adequate Airport (at a one-engine-inoperative cruise speed under standard conditions in still air) of 60 minutes for a two-engine airplane or 180 minutes for a passenger-carrying airplane with more than two engines;
(2) Within the North Polar Area; or
(3) Within the South Polar Area.
FAR 121.162 goes on to state:
Sec. 121.162 — ETOPS Type Design Approval Basis.
Except for a passenger-carrying airplane with more than two engines manufactured prior to February 17, 2015 and except for a two-engine airplane that, when used in ETOPS, is only used for ETOPS of 75 minutes or less, no certificate holder may conduct ETOPS unless the airplane has been type design approved for ETOPS and each airplane used in ETOPS complies with its CMP document as follows:
(a) For a two-engine airplane, that is of the same model airplane-engine combination that received FAA approval for ETOPS up to 180 minutes prior to February 15, 2007, the CMP document for that model airplane-engine combination in effect on February 14, 2007.
(b) For a two-engine airplane, that is not of the same model airplane-engine combination that received FAA approval for ETOPS up to 180 minutes before February 15, 2007, the CMP document for that new model airplane-engine combination issued in accordance with §25.3(b)(1) of this chapter.
(c) For a two-engine airplane approved for ETOPS beyond 180 minutes, the CMP document for that model airplane-engine combination issued in accordance with §25.3(b)(2) of this chapter.
(d) For an airplane with more than 2 engines manufactured on or after February 17, 2015, the CMP document for that model airplane-engine combination issued in accordance with §25.3(c) of this chapter.
What is the CMP? Well, that's a fun question. It is the Configuration Maintenance Procedures (CMP) document for an aircraft, and how to get an aircraft and its CMP certified for ETOPS is described in a document named "AC 120-42B - Extended Operations (ETOPS and Polar Operations)" available from the US Government here, if you're curious.
As the driver of air travel shifts from convenience, speed and luxury to cost efficiency, ETOPS has become much more important. One way that the world's fleet of jet transports has become more efficient is by moving, in all classes but not including superjumbo, from four engined designs (Boeing 707, 720, 747, Airbus 340 and 380, Douglas DC-8, Convair 880 and 990, De Havilland Comet, Vickers VC-10, Ilyushin 86 and 96, etc) and three engine tri-jets (Douglas DC-10/MD-80, Lockheed L-1011, Boeing 727, Tupolev Tu-154, etc.) to the more economical twin-engine designs. Although twin engined transports have been around since the beginning of civil aviation, recent progress in materials science and design have allowed these types to take over the mid-range and long-range routes previously only attainable with larger aircraft. Modern twinjets, paired with modern high-bypass turbofan engines, have demonstrated performance equal to or surpassing their more numerous powerplant brethren of years past.
The big limitation of twin jets up until recently has been that jet engines haven't been powerful enough, and aircraft control systems haven't been capable enough, to let aircraft like these fly for extended distances on a single engine - so if an engine failed in flight, twin engine aircraft had both very limited range and became very unstable due to the highly unbalanced thrust produced by wing-mounted twin engines. That consideration was one of the reason that early twinjet and trijets had engines clustered on the fuselage at the tail; loss of an engine produced a much less imbalanced thrust than having under-wing nacelles. Recent airplanes with advanced flight control systems are much more able to compensate for single engine thrust, and modern engines produce much higher power, allowing twin jets to begin to take on even long over-ocean routes. Boeing 767s, 777s and now 787s routinely fly transocean routes; 777s and 787s now fly the longest Pacific crossings. Airbus A310s were and are used on transatlantic routes,and Airbus A330s ply longer ones.
Two engines means less complexity, less cost, and (due to less mechanical loss by engines) generally higher fuel efficiency as well. ETOPS ratings are the benchmark for which of these more efficient aircraft can fly the longer routes. They have nearly the same range as the most recent model of the 747.