For a dispatcher of twin engined aircraft, you need to find a route that keeps you within a rule distance from diversion airports that are suitable for that aircraft and where the weather is good enough at the time when you would be in range. This is in case of loss of an engine and these alternate airports/range circles must overlap, ie. no part of the route can be outside them. They are needed mainly over oceans e.g. North Atlantic. The flight plan will show the crew when they are entering each of these range circles. I dont know what your reading material is, but this article is a good basic introduction.
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