For every route, airlines load a dozen or more different fares into reservation systems and then pick which one applies to a specific flight at a specific time, usually based on how well that flight is selling and what the airline expects in demand for seats, including demand from business travelers who book later.

WSJ: Finding the Best Fares to Europe

But many people who've spent time and money become upset when the airline can't use their miles to book the specific flight they want to take.

CNN: How can you earn frequent flier miles?