S-seals and T-seals in API 6A wellheads are elastomeric O-rings with PTFE or PEEK anti-extrusion back-ups. Used in casing spools and tubing hangers, they maintain sealability after repeated thermal cycles and high-pressure gas testing per API 6A Appendix F PR2F requirements https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/