If you press the F1 key when in that dialogue, you can read the explanation from the online Help :
"When the Use Special One-Way Load Distribution checkbox is checked, the load applied to area objects with this section property is to be distributed as a one-way slab spanning in the local 1 direction of the area object"
I'm not sure where you would use this option for walls, but for one-way slabs you could control load distribution, although I've had problems getting this option to work. Typically I end up using membrane "deck" type elements or using property modifiers and mesh techniques on slab type areas for one-way. The membrane deck seems to work better.
