There's no specific booking class for E+. UA elites (1K, 1P, 2P, 3P) get E+ seating automatically - others either have to purchase an annual E+ "subscription," pay for E+ at booking or check-in, or get lucky in the case where E- if full and the only available seats for assignment (on the day of flight) are in E+.

For JFK-LAX, all economy seats are E+ although the pitch is a little less than is typical on non-ps mainline aircraft.