It is one of those wannabes, one has to look hard to find the figure. It is probably there, but we do not see it. A. Square foot is a two dimensional measure. The thickness of material is not taken into account. It is equal to the area of the material and units are generally a square foot.
Board foot is a volumemetric measure and is generally used for lumber.
It means a piece 1 ft. Except for the USA, the whole world uses cubic meters as measurement of lumber. A. Lumber is sold in price per board foot for various thicknesses.