When purchasing Nitinol sheets, the most vital specification is the Transformation Temperature (Af point), which determines whether the material exhibits superelasticity or the shape memory effect at operating temperatures. You must also specify the chemical composition (typically 54.5%-57% Nickel) to ensure compliance with ASTM F2063 standards for medical devices or industrial use. Additionally, verify the surface finish, such as black oxide, pickled, or polished, as this affects fatigue resistance and biocompatibility.