I may have stated it awkardly, for which I apologize, but what I meant is that there are many ways for you to end up in a J/F class seat.

~ Paid with miles (from any class fare)
~ Buying a J class ticket (no award inventory as far as i know, if there are seats in that class, youre in)
~ Involantary Upgrades
~ Upgrades for delays / inconviences on flights
~ Upgrades because of special offers
~ Volantary upgrades w/ seat on later flight because they have overbooked

These are also present on other airlines, but IMHO are much more common / frequent on EK

Cheers