There's a discrepancy between the regs and OPS manuals regarding the definition of en route alternate. The EU-OPS says it's an airdrome which might be required at the planning stage, very ambiguous in my eyes. AI manual for instance says
En-route alternate: An aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition while en route.
, quite better. Regarding planning minimum, it's applicable to all of them including en route alternate, as per EU OPS 1.192
En-route alternates, 3% ERA, destination alternates and isolated destination airport. This is theory in practice you get a folder with actual and forecast weather for a bunch of airdromes along the route to have an idea where to go in case needed. As usual sound airman-ship is required. Hope that helps.
Cheers
