API 6B, by the American Petroleum Institute, specifies dimensions, materials, testing, and pressure ratings for flanges, fittings, and valves in oil/gas upstream (e.g., wellheads, manifolds). It ensures reliability in moderate-pressure applications.

API 6BX is its high-pressure counterpart, designed for extreme conditions like high-pressure/high-temperature (HPHT) wells. It has stricter requirements (e.g., enhanced material toughness, tighter tolerances) and higher pressure ratings than 6B.

Both are widely used in upstream oil/gas, ensuring interchangeability, safety, and performance of critical connections, with 6BX addressing more demanding operational needs.