Originally Posted by
evthu
I see. I have never heard that term before. Can you explain more?
It's exactly what you found. United is willing to sell a ticket from PHL-AMS via FRA, but not a flight simple from PHL-FRA. Airline fares are based on demand between the endpoints without regard for intermediate points - more people want to fly from PHL-FRA than from PHL-AMS, so inventory is adjusted appropriately to make it more expensive to fly PHL-FRA, and you can only get the cheap PHL-FRA flight if you continue on to a "less desirable" city. It doesn't normally apply to awards, but this is not unheard of.
If you search FT (or Google), you will find lots more about married segments.