I think this ambiguity results from the interplay of two different National benefits.

Aisle access requires a midsize rate. If you're EC, you get access to the Emerald Aisle; if you're Exec or EE, you get access to the Executive Selection.

Additionally, Exec and EE have the ability to reserve a full size through luxury car at a one-rate discount. That is to say: a reservation for the full size class is charged at the midsize rate; likewise, a premium reservation is charged at the full size rate, and a luxury reservation is charged at the premium rate. There is, however, no discount for reserving a midsize car.

As a result -- if you're Exec or EE, reserving either a full size car or a midsize car results in a reservation with the midsize rate, though the reserved class differs. And since the rate is midsize, you get access to your designated aisle.

Of course, this only works at an Aisle location. At counter service locations, IME reserving the full size is almost always going to get you a nicer car.