What’s the Differences Between Flanges’ Different Joint Faces-- FF and RF?

What's the differences between flanges'different joint faces-- FF and RF?

RF is raised face flange, which can be used under the pressure of PN1.6~PN10MPA.
FF is flat face flange, which has lower sealability than RF, its pressure is 1.6~2.0MPA (standard: GB9113), and other joint faces like RTJ, FM have higher pressure. The gaskets for RF and FF can be used for each other.