 Moreover, the conditions are fulfilled in the universal case in the sense that the map from the moduli stack of elliptic curves to the moduli stack of formal groups 

is flat.This gives then a presheaf of cohomology theories over the site of affine schemes flat over the moduli stack of elliptic curves.The desire to get a universal elliptic cohomology theory by taking global sections has led to the construction of the topological modular forms.