What is the connection method of SAE flange?

Some pipe fittings and equipment have their own SAE flanges, which are also SAE flange connections. SAE flange connection is an important connection mode in pipeline construction. The six types of SAE flange connection are as follows: A-flange connection, that is, single flange (with O-ring), and the other end is equipment. B-flange coupling, that is, paired flanges (one with an O-ring). Type C-Flanged coupling without flange welding neck. I don't know much about this type and don't know how to use it.