This specification establishes design validation requirements for API Specification 6A surface safety valves/underwater safety valves (SSV/USV) and associated valve bore sealing mechanism(s) for Class II and Class III. These classes are intended for use if substances such as sand can be expected to cause an SSV/USV valve failure. Class III adds requirements for the validation of the valve bonnet assembly inclusive of stem seals and may be selected by the user/purchaser. Validation to Class III also validates the same SSV/USV for Class II in accordance with scaling limitations specified in the document.The validation requirements in this specification are not represented as duplicating actual well conditions.