If your material is not listed in that code (Appendix A), then you are using an "unlisted" material. Para. 123.1.2 states some requirements for that material. That paragraph refers you back to Para. 102.3.1(c) for setting allowable stress (Sc & Sh), but that points you to ASME Boiler & Pressure Vesel Code Section II, Part D, Appendix 1. I believe you can use the lower of (1/3.5) times the ultimate tensile strength or 2/3 yield strength. Other criteria, such as creep and rupture strength may apply.
Check the code for a complete definition.