For practical purposes, none, to my knowledge. My impression was ASME puts an "S" before an ASTM specification to say it's then an "ASME BPVC" material spec. Each of these SA designations is listed in ASME BPVC (Boiler and Pressure Vessel Code), Section II, Part A, Appendix A, I think, along with any deviation from the corresponding ASTM spec.; and most (but not all) of them say "identical to ASTM spec.," or essentially identical. I think the BPVC experts are at forum292.