384It follows from those considerations that the assessment of the substitutability of two products is not limited to determining whether those products are, from a functional point of view, capable of satisfying the same need, but also requires a determination as to whether, from an economic point of view, those products are in fact substitutable. Economic substitutability between two products may be found where changes in their relative prices lead to a shift in the sales of one to the other. In that regard, it must be pointed out that, as is apparent from paragraph 13 of the Notice on market definition, referred to in paragraph 2 of this judgment and to which the General Court made reference in paragraph 1384 of the judgment under appeal, from an economic point of view, demand substitutability constitutes the most immediate and effective disciplinary constraint on the suppliers of a given product. The assessment of that substitutability consists essentially in assessing the cross-price elasticity of demand by determining whether consumers of a product subject to a small but permanent price increase would switch to substitute products.