Material classes within API 6A designate seal chemical compatibility with specific well environments including sweet gas, sour gas with hydrogen sulfide, and carbon dioxide service. Classes AA through HH identify material formulations resistant to particular corrosive conditions, preventing premature seal degradation from chemical attack that would compromise well integrity and safety. https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/