The raised face is referred to as a raised face because the gasket surfaces are raised above the bolting circle face.
The main purpose of the raised face is to concentrate more pressure on a smaller gasket area and thereby increase the pressure containment capability of the joint.
The height of the raised face varies depending on the size but generally it is 2~7mm.

Additional Information
Compared with FF and RF, RTJ, T&G and M&F provide stronger connection and better fit. However, they are more expensive than FF and RF, and commercial availabilty can be limited since FF and RF are most commonly used in the market.
In addition, when using T&G and/or M&F, the complexity of designing the piping system may increase since they have to be matched.
Same surface type should be used and it is not recommended to assemble with a different surface type (e.g., assembling RTJ with RF).
When not using the same surface type, a gap may exist in the gasket part and a trustable connection cannot be guaranteed, and thereby a leakage may occur.