Gaskets are also called ring joints. They have R, RX and BX three types. 

Type R,Oval shape applys to standard groove flange.

Type R,octagonal shape applys to standard groove flange.

Type RX, the pressure can up to approximately 700bar. To using this kind of gaskets firstly the outer sealing surface approach to flange, the higher system pressure generates higher bottom stress. The type RX can interchangeable with standard type R. 

Type BX, the pressure can up to approximately 1500bar. BX gaskets are self energized by the internal system pressure which increases the gasket seating stress on the flange contact surface. This Ring Joint is only suited for API-type BX flanges and grooves.