Not true... if the flight is economy-only then you generally book into Y with an AONEx ticket. See here: http://www.hardlink.com/~markdu/OWFi...kingCodes.html

What is not altogether clear to me is what class you book into on an AONEx ticket if the flight offers First but there is no availability. If going down to Business then it is D, but going down to Economy would it be L or Y?