I've been thinking about this distinction and I'd say that a double pale/extra pale would be a beer with the same malt character and OG as a pale ale, but with a little extra hopping.
So a typical APA recipe might be around 1.055, 40IBUs, and have 1oz additions at 10min and 0min. An extra pale would have the exact same malt bill, 40-55IBUs, and 1.5-2oz additions at 10min and 0min as well as some dry hopping.
Technically, these probably still fit into the APA category if you were to enter them in a homebrew contest. The thing that pisses me off about contests is I feel like in the APA category you HAVE to brew an extra pale to be noticeable. Otherwise some other bastard's hoppy APA will overshadow yours. This is why I'm turning away from competitions - they make me feel bad about liking my beer!