E+ is United's front-of-coach zone with 2-5" higher pitch/legroom (or all of coach, on their transcon "p.s." flights). It is free for UA elites, for pseudo-elites (they have a sub-bottom tier called Premier Associate), for any MileagePlus member who pays a flat-fee for all-year access (Annual Option), or for anyone at all who pays a variable, per-flight supplement at booking or check-in.

On day of departure, if the rest of coach (aka "E-" in UA-board language) is full, the E+ seats are available to anyone still needing seat assignment, without paying the fee. E+ is available on all of UA mainline, and also on selected so-called "exPlus" RJ services.

UA used to allow free E+ seating at time of booking for its partner elites (like US), but removed this offer a couple of years ago. Will they allow it for CO elites? Absent some other realignments of the partner agreements, it seems doubtful at least so far.

p.s. here's a hidden benefit for your list for AA: the Priority Access seating zone (front of coach) tends to have power ports at every row (mainline only). the back of coach is only every third row. every seat in premium cabins has a power port.