A characterization of relative pure flat R-modules

Let R be a commutative ring with identity. For a given class S of R-modules, the class of all S-pure flat R-modules is covering. Let S be any class of finitely presented R-modules containing R. Then an R-module M is S-pure flat if and only if it is a direct limit of a direct system consisting of R-modules isomorphic to elements belong to add(tr(S)).