API stands for American Petroleum Institute, an organization largely engaged into developing standards for petroleum industry. It has created various dimensional standards for pipeline valves, flanges and gaskets.
DIN standards for German institute for standards, developed a variety of products including buttweld pipe fittings, flanges, valves and metric fasteners. DIN standards are widely used for flanges and fasteners.
American Water Works Association or AWWA is repsonsible for developing various products standards guiding dimensions for manufacturers and piping engineers. Most commonly referred product dimensions are for water pipeline flanges.
GOST refers to a set of technical standards maintained by Euro-Asian Council for Standardization, developed during soviet era and are used across Russian continent. It covers dimensions for pipeline flanges, valves, pipe fittings, etc.
Society for Automotive Engineers, abbreviated as SAE is an American organization, developing technical standards for various types of flanges, pipe and tube fittings, valves and high strength fasteners for automotive industry.
JIS represents Japanese Industrial Standards, quite different and having little equivalency to other internation standards. JIS developed many dimension standards for steel pipes, pipe fittings, tube fittings, flanges, valves, etc.