There might be multiple routes, belonging to the “Preferred Routes” class, connecting the same pair of airports.
However some of them might be more appealing for non-urgent circumstances such as less turbulence, more favorable winds, etc. Therefore there will be a route (as long as I understand) that is the desired one by many airlines at the same time.
Let’s suppose we have airlines A, B, and C requesting a flight on route x, but the capacity is one per route, or stated another way, only one of the three airlines can take the route and the other two must choose another route.
Which is the protocol followed to decide if the route x is assigned to A, B, or C?