Also a consideration is specific equipment that an individual aircraft is required to have operable for certain types of airspace. For example most oceanic airspace has a list of requirements including things like HF radios. If an aircraft is dispatched with an MEL item, the flight planning software will be able to filter available airspace based on this and this might generate a very different route. Two 777s for example might have to take different routes at the same time based on equipment that's available on board.