Alternate Enroute Airports or alternate airports
As in the ATC flight plan you would normally only enter in the Alternates and the Reclear airports and an alternate enroute airport is usually only in the OFP.
the alternate enroute airport depaends on many factors, namely:
1. Aircraft performance
2. On route MORA
3. Airport facilities
As a rule of thumb if your not ETOPs certified the enroute alternate would be the one hour rule from flight path, but this could be varied if the terrain clearance on route for the drift down profile doesn't accept it.
Also another important criteria for selecting your enroute alternate is the fact that the airport should be acceptable to land on for your aircraft performance and the emergency services provided by the selected airport.
Hope this helps