deja vu ?
it doesn't take much to confuse me, and you just did it
I just posted this in the other thread . . .
A-DEP looks at all of the focus points (they all get used even if switched off by manual selection) and selects an aperture (with suitable shutter speed) to ensure that all the focus points are within the depth of field.
I find it useful occasionally, especially if there is a big depth of field.
Last time I used it was a large area (motorbike club stand at a show) where I actually just held the camera up high above my head, pointing in roughly the right direction on widest 18mm and hit the button. Crude but effective.